build: try to ban GNU extensions

[compnerd]

GNU extensions are inherently non-portable, and can cause regressions on
other platforms. Attempt to catch the accidental use of GNU extensions
early by using clang to identify them.

Replace this paragraph with a description of your changes and rationale. Provide links to external references/discussions if appropriate.

Resolves SR-NNNN.

[compnerd]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 06dd33c

[DougGregor]

Good idea. Does LLVM already do this? Or are we trying to stage it in here first, then do it upstream in LLVM?

[compnerd]

I don't think that LLVM adds this particular flag. I can certainly bring this up for LLVM, but, its less likely to be an issue since the CI there will email you if the change breaks on MSVC.

[CodaFi]

Hmph, good that we know it works...

[compnerd]

sigh who let me near a computer? of course this is going to fail! I just fixed the instance of the GNU extension!

[compnerd]

@swift-ci please test

[jrose-apple]

A little worried about us being more strict than LLVM because of headers, but okay.

[compnerd]

@jrose-apple - yeah, I figure that if we can get Swift to build with -Werror=gnu I can try to add that to LLVM upstream (and then pull that back down).