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

Preprocessing the preprocessor output again interacts poorly with some
flag combinations when we perform a separate preprocessing stage. In our
case, `-no-integrated-cpp -dD` triggered this issue; but I guess that
other flags could also trigger problems (`-save-temps` instead of
`-no-integrated-cpp`).

Full context (which is quite weird I'll admit):

* To cache OpenCL kernel compilation results, we use the
`-no-integrated-cpp` for the driver to generate a separate preprocessing
command (`clang -E`) before the rest of the compilation.
* Some OpenCL C language features are implemented as macro definitions
(in `opencl-c-base.h`). The semantic analysis queries the preprocessor
to check if these are defined or not, for example, when we checks if a
builtin is available when using `-fdeclare-opencl-builtins`.
* To preserve these `#define` directives, on the preprocessor's output,
we use `-dD`. However, other `#define` directives are also maintained
besides OpenCL ones; which triggers the issue shown in this PR.

A better fix for our particular case could have been to move the
language features implemented as macros into some sort of a flag to be
used together with `-fdeclare-opencl-builtins`.
But I also thought that not preprocessing preprocessor outputs seemed
like something desirable. I hope to work on this on a follow up.

1 files changed, 3 insertions, 1 deletions

diff --git a/clang/lib/CodeGen/CodeGenAction.cpp b/clang/lib/CodeGen/CodeGenAction.cpp
index eb5b604..2c0767f 100644
--- a/clang/lib/CodeGen/CodeGenAction.cpp
+++ b/clang/lib/CodeGen/CodeGenAction.cpp
@@ -908,6 +908,8 @@ bool CodeGenAction::loadLinkModules(CompilerInstance &CI) {
 bool CodeGenAction::hasIRSupport() const { return true; }
 
 void CodeGenAction::EndSourceFileAction() {
+  ASTFrontendAction::EndSourceFileAction();
+
   // If the consumer creation failed, do nothing.
   if (!getCompilerInstance().hasASTConsumer())
     return;
@@ -932,7 +934,7 @@ CodeGenerator *CodeGenAction::getCodeGenerator() const {
 bool CodeGenAction::BeginSourceFileAction(CompilerInstance &CI) {
   if (CI.getFrontendOpts().GenReducedBMI)
     CI.getLangOpts().setCompilingModule(LangOptions::CMK_ModuleInterface);
-  return true;
+  return ASTFrontendAction::BeginSourceFileAction(CI);
 }
 
 static std::unique_ptr<raw_pwrite_stream>