[HLSL] Fix vector list initialization (#161421)
This simplifies and cleans up the vector list initialization behavior.
This simplifies the work we do in SemaInit by just relying on SemaHLSL's
initialization list flattening. This change fixes some outstanding
limitations by supporting structure to vector initialization, but
re-introduces HLSL's limitations around overload resolution in
initializers.
---------
Co-authored-by: Helena Kotas <hekotas@microsoft.com>
