[lldb] Fix error : unknown error while starting lldb's C/C++ repl (llvm#153560)

Fixes llvm#153157

The proposed solution has been discussed here
(llvm#153157 (comment))

This is what we would be seeing now 

```
base) anutosh491@Anutoshs-MacBook-Air bin % ./lldb /Users/anutosh491/work/xeus-cpp/a.out
(lldb) target create "/Users/anutosh491/work/xeus-cpp/a.out"
Current executable set to '/Users/anutosh491/work/xeus-cpp/a.out' (arm64).
(lldb) b main
Breakpoint 1: where = a.out`main, address = 0x0000000100003f90
(lldb) r
Process 71227 launched: '/Users/anutosh491/work/xeus-cpp/a.out' (arm64)
Process 71227 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
    frame #0: 0x0000000100003f90 a.out`main
a.out`main:
->  0x100003f90 <+0>:  sub    sp, sp, #0x10
    0x100003f94 <+4>:  str    wzr, [sp, #0xc]
    0x100003f98 <+8>:  str    w0, [sp, #0x8]
    0x100003f9c <+12>: str    x1, [sp]
(lldb) expression --repl -l c -- 
  1> 1 + 1
(int) $0 = 2
  2> 2 + 2
(int) $1 = 4
```

```
base) anutosh491@Anutoshs-MacBook-Air bin % ./lldb /Users/anutosh491/work/xeus-cpp/a.out
(lldb) target create "/Users/anutosh491/work/xeus-cpp/a.out"
Current executable set to '/Users/anutosh491/work/xeus-cpp/a.out' (arm64).
(lldb) b main
Breakpoint 1: where = a.out`main, address = 0x0000000100003f90
(lldb) r
Process 71355 launched: '/Users/anutosh491/work/xeus-cpp/a.out' (arm64)
Process 71355 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
    frame #0: 0x0000000100003f90 a.out`main
a.out`main:
->  0x100003f90 <+0>:  sub    sp, sp, #0x10
    0x100003f94 <+4>:  str    wzr, [sp, #0xc]
    0x100003f98 <+8>:  str    w0, [sp, #0x8]
    0x100003f9c <+12>: str    x1, [sp]
(lldb) expression --repl -l c -- 3 + 3
Warning: trailing input is ignored in --repl mode
  1> 1 + 1
(int) $0 = 2
```

Author: anutosh491

lldb/source/Commands/CommandObjectExpression.cpp

@@ -640,9 +640,15 @@ void CommandObjectExpression::DoExecute(llvm::StringRef command,
             repl_sp->SetValueObjectDisplayOptions(m_varobj_options);
           }
 
+          if (!expr.empty()) {
+            result.GetOutputStream().Printf(
+                "Warning: trailing input is ignored in --repl mode\n");
+          }
+
           IOHandlerSP io_handler_sp(repl_sp->GetIOHandler());
           io_handler_sp->SetIsDone(false);
           debugger.RunIOHandlerAsync(io_handler_sp);
+          return;
         } else {
           repl_error = Status::FromErrorStringWithFormat(
               "Couldn't create a REPL for %s",

lldb/test/API/repl/clang/TestClangREPL.py

@@ -56,6 +56,16 @@ def test_basic_completion(self):
 
         self.quit()
 
+        # Re-enter the REPL with trailing input to trigger warning.
+        self.child.send("expression --repl -l c -- 3 + 3\n")
+        self.child.expect_exact("Warning: trailing input is ignored in --repl mode\n")
+        self.child.expect_exact("1>")
+
+        # Evaluate another expression after warning.
+        self.expect_repl("4 + 4", substrs=["(int) $3 = 8"])
+
+        self.quit()
+
     # PExpect uses many timeouts internally and doesn't play well
     # under ASAN on a loaded machine..
     @skipIfAsan