[clang] [HLSL] Convert vectors to bool for unary ! (PR #163857)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

We definitely do similar stuff for our ExtVector type, so I don't think of this as horribly inconsistent with the language.  However, the base-`vector` type is 'owned' by GCC (Clang's version is the ExtVectorType), so I don't think we should be modifying the GCC version unless they are going to do the same.  At that point, I don't think it should be HLSL specific.

That said, extending this 'splat-esque' behavior for ExtVectorType to HLSL should be non-controversial.

https://github.com/llvm/llvm-project/pull/163857