[clang] [Preprocessor] Do not expand macros if the input is already preprocessed (PR #137665)

[Juan Manuel Martinez Caamaño via cfe-commits]

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@@ -84,6 +84,8 @@ class FrontendAction {
   /// \return True on success; on failure ExecutionAction() and
   /// EndSourceFileAction() will not be called.
   virtual bool BeginSourceFileAction(CompilerInstance &CI) {
+    if (CurrentInput.isPreprocessed())
+      CI.getPreprocessor().SetEnableMacroExpansion(false);
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jmmartinez wrote:

Use `SetMacroExpansionOnlyInDirectives` instead. We still need to expand macro ids in #pragma directives when going through preprocessed code. Usage of macros on #pragma directives are not expanded in the preprocessed source.

https://github.com/llvm/llvm-project/pull/137665