[Clang] constexpr support for __builtin_convertvector and __builtin_shufflevector

[philnik777]

LanguageExtensions.rst claims that __builtin_shufflevector and __builtin_convertvector can be constant evaluated, but that's not reflected inside Butiltins.td.

[zwuis]

This is part of #46593.

[RKSimon]

This was fixed by #76615

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Nikolas Klauser (philnik777)

LanguageExtensions.rst claims that `__builtin_shufflevector` and `__builtin_convertvector` can be constant evaluated, but that's not reflected inside `Butiltins.td`.

[philnik777]

@RKSimon No, it hasn't: https://godbolt.org/z/c3bTe5Y8E. This report is about the inconsistency that it's documented as such inside LanguageExtensions.rst but not actually marked as such, i.e. a Constexpr attribute is missing from Builtins.td.