[HLSL] Support HLSL vector initializers

In HLSL vectors are ext_vectors in all respects except that they
support a constructor style syntax for initializing vectors. This
change adds a translation of vector constructor arguments into
initializer lists.

This supports two oddities of HLSL syntax:
(1) HLSL vectors support constructor syntax
(2) HLSL vectors are expanded to constituate components in constructors

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D127802