[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().SetDisableMacroExpansion();
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jmmartinez wrote:

It was quite shortsighted from me, sorry. There were some subclasses that were missing the call to the base-class method.

I should add the calls to the base class in a separate NFC patch.

I've added a `SetEnableMacroExpansion(true/false)` method to set/reset the macro expansion; but I'm not convinced by it (I'm worried about interactions with `SetMacroExpansionOnlyInDirectives`, but these interactions also seem unlikely).

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