[PATCH] D121099: [C++20][Modules][HU 5/5] Add fdirectives-only mode for preprocessing output.

[Iain Sandoe via Phabricator via cfe-commits]

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When the -fdirectives-only option is used together with -E, the preprocessor
output reflects evaluation of if/then/else directives.

As such, it preserves defines and undefs of macros that are still live after
such processing.  The intent is that this output could be consumed as input
to generate considered a C++20 header unit.

We strip out any (unused) defines that come from built-in, built-in-file or
command line; these are re-added when the preprocessed source is consumed.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D121099

Files:
  clang/include/clang/Driver/Options.td
  clang/include/clang/Frontend/PreprocessorOutputOptions.h
  clang/lib/Frontend/CompilerInvocation.cpp
  clang/lib/Frontend/PrintPreprocessedOutput.cpp
  clang/test/Modules/cxx20-hu-05.cpp

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