[clang-repl] fix error recovery while parsing completely fails (#127087)

Fixes the following crash in clang-repl

```c++
clang-repl> try { throw 1; } catch { 0; }
In file included from <<< inputs >>>:1:
input_line_1:1:23: error: expected '('
    1 | try { throw 1; } catch { 0; }
      |                       ^
      |                       (
clang-repl: /home/vipul-cariappa/Documents/Workspace/cpp-py/llvms/llvm-project-a/clang/lib/AST/DeclBase.cpp:1757: void clang::DeclContext::addHiddenDecl(clang::Decl*): Assertion `D->getLexicalDeclContext() == this && "Decl inserted into wrong lexical context"' failed.
 #0 0x000059b28459e6da llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) /home/vipul-cariappa/Documents/Workspace/cpp-py/llvms/llvm-project-a/llvm/lib/Support/Unix/Signals.inc:804:22
 #1 0x000059b28459eaed PrintStackTraceSignalHandler(void*) /home/vipul-cariappa/Documents/Workspace/cpp-py/llvms/llvm-project-a/llvm/lib/Support/Unix/Signals.inc:880:1
 #2 0x000059b28459bf7f llvm::sys::RunSignalHandlers() /home/vipul-cariappa/Documents/Workspace/cpp-py/llvms/llvm-project-a/llvm/lib/Support/Signals.cpp:105:20
 #3 0x000059b28459df8e SignalHandler(int, siginfo_t*, void*) /home/vipul-cariappa/Documents/Workspace/cpp-py/llvms/llvm-project-a/llvm/lib/Support/Unix/Signals.inc:418:13
 #4 0x000077cdf444ea50 (/usr/lib/libc.so.6+0x42a50)
 #5 0x000077cdf44aee3b pthread_kill (/usr/lib/libc.so.6+0xa2e3b)
 #6 0x000077cdf444e928 raise (/usr/lib/libc.so.6+0x42928)
 #7 0x000077cdf443156c abort (/usr/lib/libc.so.6+0x2556c)
 #8 0x000077cdf44314d2 __assert_perror_fail (/usr/lib/libc.so.6+0x254d2)
 #9 0x000077cdf4444c56 (/usr/lib/libc.so.6+0x38c56)
#10 0x000059b28495bfc4 clang::DeclContext::addHiddenDecl(clang::Decl*) /home/vipul-cariappa/Documents/Workspace/cpp-py/llvms/llvm-project-a/clang/lib/AST/DeclBase.cpp:1759:3
#11 0x000059b28495c0f5 clang::DeclContext::addDecl(clang::Decl*) /home/vipul-cariappa/Documents/Workspace/cpp-py/llvms/llvm-project-a/clang/lib/AST/DeclBase.cpp:1785:37
#12 0x000059b28773cc2a clang::Sema::ActOnStartTopLevelStmtDecl(clang::Scope*) /home/vipul-cariappa/Documents/Workspace/cpp-py/llvms/llvm-project-a/clang/lib/Sema/SemaDecl.cpp:20302:18
#13 0x000059b286f1efdf clang::Parser::ParseTopLevelStmtDecl() /home/vipul-cariappa/Documents/Workspace/cpp-py/llvms/llvm-project-a/clang/lib/Parse/ParseDecl.cpp:6024:62
#14 0x000059b286ef18ee clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) /home/vipul-cariappa/Documents/Workspace/cpp-py/llvms/llvm-project-a/clang/lib/Parse/Parser.cpp:1065:35
#15 0x000059b286ef0702 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&, clang::Sema::ModuleImportState&) /home/vipul-cariappa/Documents/Workspace/cpp-py/llvms/llvm-project-a/clang/lib/Parse/Parser.cpp:758:36
#16 0x000059b28562dff2 clang::IncrementalParser::ParseOrWrapTopLevelDecl() /home/vipul-cariappa/Documents/Workspace/cpp-py/llvms/llvm-project-a/clang/lib/Interpreter/IncrementalParser.cpp:66:36
#17 0x000059b28562e5b7 clang::IncrementalParser::Parse(llvm::StringRef) /home/vipul-cariappa/Documents/Workspace/cpp-py/llvms/llvm-project-a/clang/lib/Interpreter/IncrementalParser.cpp:132:8
#18 0x000059b28561832b clang::Interpreter::Parse(llvm::StringRef) /home/vipul-cariappa/Documents/Workspace/cpp-py/llvms/llvm-project-a/clang/lib/Interpreter/Interpreter.cpp:570:8
#19 0x000059b285618cbd clang::Interpreter::ParseAndExecute(llvm::StringRef, clang::Value*) /home/vipul-cariappa/Documents/Workspace/cpp-py/llvms/llvm-project-a/clang/lib/Interpreter/Interpreter.cpp:649:8
#20 0x000059b2836f9343 main /home/vipul-cariappa/Documents/Workspace/cpp-py/llvms/llvm-project-a/clang/tools/clang-repl/ClangRepl.cpp:255:59
#21 0x000077cdf443388e (/usr/lib/libc.so.6+0x2788e)
#22 0x000077cdf443394a __libc_start_main (/usr/lib/libc.so.6+0x2794a)
#23 0x000059b2836f7965 _start (./bin/clang-repl+0x73b8965)
fish: Job 1, './bin/clang-repl' terminated by signal SIGABRT (Abort)
```

With this change:
```c++
clang-repl> try { throw 1; } catch { 0; }
In file included from <<< inputs >>>:1:
input_line_1:1:23: error: expected '('
    1 | try { throw 1; } catch { 0; }
      |                       ^
      |                       (
error: Parsing failed.
clang-repl> 1;
clang-repl> %quit
```

Author: Vipul-Cariappa

clang/lib/Parse/ParseDecl.cpp

@@ -6024,7 +6024,7 @@ Parser::DeclGroupPtrTy Parser::ParseTopLevelStmtDecl() {
   TopLevelStmtDecl *TLSD = Actions.ActOnStartTopLevelStmtDecl(getCurScope());
   StmtResult R = ParseStatementOrDeclaration(Stmts, SubStmtCtx);
   if (!R.isUsable())
-    return nullptr;
+    R = Actions.ActOnNullStmt(Tok.getLocation());
 
   Actions.ActOnFinishTopLevelStmtDecl(TLSD, R.get());
 

clang/unittests/Interpreter/InterpreterTest.cpp

@@ -107,6 +107,13 @@ TEST_F(InterpreterTest, Errors) {
 
   auto RecoverErr = Interp->Parse("int var1 = 42;");
   EXPECT_TRUE(!!RecoverErr);
+
+  Err = Interp->Parse("try { throw 1; } catch { 0; }").takeError();
+  EXPECT_THAT(DiagnosticOutput, HasSubstr("error: expected '('"));
+  EXPECT_EQ("Parsing failed.", llvm::toString(std::move(Err)));
+
+  RecoverErr = Interp->Parse("var1 = 424;");
+  EXPECT_TRUE(!!RecoverErr);
 }
 
 // Here we test whether the user can mix declarations and statements. The