[clang] Expand invalid PCM diagnostic (#69489)

Summary:

When a PCM file is loaded, it can go wrong in various ways. The current
diagnostic only produces the name of the malformed PCM, not why it is
malformed. Expand the diagnostic to display what went wrong!

There is only one call site for this diagnostic, and it already passes
the error message:


https://github.com/llvm/llvm-project/blob/main/clang/lib/Serialization/ASTReader.cpp#L4763-L4764

Test Plan:

The modified LIT test.

---------

Co-authored-by: Nuri Amari <nuriamari@fb.com>

Author: NuriAmari

clang/include/clang/Basic/DiagnosticSerializationKinds.td

@@ -62,7 +62,7 @@ def err_ast_file_out_of_date : Error<
   "%select{PCH|module|AST}0 file '%1' is out of date and "
   "needs to be rebuilt%select{|: %3}2">, DefaultFatal;
 def err_ast_file_invalid : Error<
-  "file '%1' is not a valid precompiled %select{PCH|module|AST}0 file">, DefaultFatal;
+  "file '%1' is not a valid precompiled %select{PCH|module|AST}0 file: %2">, DefaultFatal;
 def note_module_file_imported_by : Note<
   "imported by %select{|module '%2' in }1'%0'">;
 def err_module_file_not_module : Error<

clang/test/Modules/explicit-build.cpp

@@ -161,7 +161,7 @@
 // RUN:            -fmodule-file=%t/not.pcm \
 // RUN:            %s 2>&1 | FileCheck --check-prefix=CHECK-BAD-FILE %s
 //
-// CHECK-BAD-FILE: fatal error: file '{{.*}}not.pcm' is not a valid precompiled module file
+// CHECK-BAD-FILE: fatal error: file '{{.*}}not.pcm' is not a valid precompiled module file: file too small to contain AST file magic
 
 // RUN: not %clang_cc1 -x c++ -std=c++11 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -Rmodule-build -fno-modules-error-recovery \
 // RUN:            -fmodule-file=%t/nonexistent.pcm \