[PATCH] D135045: [Frontend] Recognize environment variable SOURCE_DATE_EPOCH

[Fangrui Song via Phabricator via cfe-commits]

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See https://reproducible-builds.org/docs/source-date-epoch/ . The environment
variable has ``SOURCE_DATE_EPOCH`` been recognized by many compilers.

In GCC, if `SOURCE_DATE_EPOCH` is set, it specifies a UNIX timestamp to be used
in replacement of the current date and time in the `__DATE__` and `__TIME__`
macros. Note: `__TIMESTAMP__` uses the modification time of the current source
file, which is unaffected by `SOURCE_DATE_EPOCH`.

The patch implements the GCC behavior.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D135045

Files:
  clang/docs/ReleaseNotes.rst
  clang/include/clang/Basic/DiagnosticFrontendKinds.td
  clang/include/clang/Lex/PreprocessorOptions.h
  clang/lib/Frontend/CompilerInvocation.cpp
  clang/lib/Lex/PPMacroExpansion.cpp
  clang/test/Preprocessor/SOURCE_DATE_EPOCH.c

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