[Clang] Implement CWG2518 - static_assert(false)

[hahnjo]

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This allows static_assert(false) to not be ill-formed in template definitions.

This change is applied as a DR in all C++ modes.

Of notes, a couple of tests were relying of the eager nature of static_assert

• test/SemaTemplate/instantiation-dependence.cpp
• test/SemaTemplate/instantiate-var-template.cpp

I don't know if the changes to static_assert still allow that sort of tests to be expressed.

Reviewed By: #clang-language-wg, erichkeane, aaron.ballman

Differential Revision: https://reviews.llvm.org/D144285

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Fixes the build with newer versions of MSVC's STL, reported as https://github.com/root-project/root/issues/15321

[github-actions[bot]]

Test Results

    11 files      11 suites   2d 13h 44m 43s :stopwatch:  2 645 tests  2 645 :white_check_mark: 0 :zzz: 0 :x: 27 455 runs  27 455 :white_check_mark: 0 :zzz: 0 :x:

Results for commit b6f480eb.

:recycle: This comment has been updated with latest results.

[StephanTLavavej]

We've verified that your patch fixes Root with our in-development compiler and STL, thanks!