[clang][Preprocessor] Handle the first pp-token in EnterMainSourceFile (#145244)

Depends on [[clang][Preprocessor] Add peekNextPPToken, makes look ahead
next token without
side-effects](https://github.com/llvm/llvm-project/pull/143898).

This PR fix the performance regression that introduced in
https://github.com/llvm/llvm-project/pull/144233.
The original PR(https://github.com/llvm/llvm-project/pull/144233) handle
the first pp-token in the main source file in the macro
definition/expansion and `Lexer::Lex`, but the lexer is almost always on
the hot path, we may hit a performance regression. In this PR, we handle
the first pp-token in `Preprocessor::EnterMainSourceFile`.

---------

Signed-off-by: yronglin <yronglin777@gmail.com>

1 files changed, 3 insertions, 5 deletions

diff --git a/clang/lib/Sema/SemaModule.cpp b/clang/lib/Sema/SemaModule.cpp
index fe70ce3..7c982bc 100644
--- a/clang/lib/Sema/SemaModule.cpp
+++ b/clang/lib/Sema/SemaModule.cpp
@@ -337,11 +337,9 @@ Sema::ActOnModuleDecl(SourceLocation StartLoc, SourceLocation ModuleLoc,
   // tokens in a file (excluding the global module fragment.).
   if (getLangOpts().CPlusPlusModules && !IntroducerIsFirstPPToken && !SeenGMF) {
     Diag(ModuleLoc, diag::err_module_decl_not_at_start);
-    SourceLocation BeginLoc = PP.getMainFileFirstPPToken().getLocation();
-    if (BeginLoc.isValid()) {
-      Diag(BeginLoc, diag::note_global_module_introducer_missing)
-          << FixItHint::CreateInsertion(BeginLoc, "module;\n");
-    }
+    SourceLocation BeginLoc = PP.getMainFileFirstPPTokenLoc();
+    Diag(BeginLoc, diag::note_global_module_introducer_missing)
+        << FixItHint::CreateInsertion(BeginLoc, "module;\n");
   }
 
   // C++23 [module.unit]p1: ... The identifiers module and import shall not