[Clang] Support elementwise/reduction builtins in constexpr contexts.

[fhahn]

Bugzilla Link	52445
Version	trunk
OS	All
CC	@DougGregor,@RKSimon,@zygoloid

Extended Description

Clang recently specified a set of builtins for elementwise operations and reductions on vectors:

https://clang.llvm.org/docs/LanguageExtensions.html#vector-builtins

They should also be supported in constexpr contexts.

[llvmbot]

Hi!

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[dlumma]

This issue was submitted some time ago, but it looks like no one has claimed it, so I will make an attempt to contribute here.

[RKSimon]

@dlumma Any progress?

[RKSimon]

@dlumma reverse-ping

[RKSimon]

@dlumma It doesn't look like you've made any progress on this in the past year so I've removed your assignment in case anybody else wanted to try - I know @c8ef has shown an interest in fixing this.