[RFC PATCH] parse #pragma GCC diagnostic in libcpp

[Manuel LÃpez-IbÃÃez <lopezibanez at gmail dot com>]

Currently, #pragma GCC diagnostic is handled entirely by the FE. This
has several drawbacks:

* PR c++/53431 - C++ preprocessor ignores #pragma GCC diagnostic: The
C++ parser lexes (and preprocesses) before handling the pragmas.

* PR 53920 - "gcc -E" does not honor #pragma GCC diagnostic ignored
"-Wunused-macro": Because -E does not invoke the FE code that parses
the FE pragmas.

* PR 64698 - preprocessor ignores #pragma GCC diagnostic when using
-save-temps. Same issue as above.

The following patch moves the handling of #pragma GCC diagnostic to
libcpp but keeps the interface with the diagnostic machinery in the FE
by using a call-back function.

One serious problem with this approach is that the preprocessor will
delete the pragmas from the preprocessed output, thus '-E',
'-save-temps'  will not contain the pragmas and compiling the
preprocessed file will trigger the warnings that they were meant to
suppress.  Any ideas how to prevent libcpp from deleting the #pragmas?

No Changelog since this is not a request for approval, but comments are welcome.

Cheers,

Manuel.

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