[clang] [clang] Optimize Lexer hot path to reduce compile time (PR #177153)

[Corentin Jabot via cfe-commits]

================
@@ -4539,6 +4518,9 @@ bool Lexer::LexTokenInternal(Token &Result, bool TokAtPhysicalStartOfLine) {
   return true;
 
 HandleDirective:
+  // We parsed a # character and it's the start of a preprocessing directive.
+
+  FormTokenWithChars(Result, CurPtr, tok::hash);
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cor3ntin wrote:

```suggestion
  // We parsed a # character which is the start of a preprocessing directive.
```

https://github.com/llvm/llvm-project/pull/177153