HLSL doesn't support access specifiers. This change has two components:
1) Make default access for classes public
2) Diagnose the use of access specifiers as a clang HLSL extension
As long as the default behavior for access specifiers matches HLSL,
allowing them to be used doesn't cause sourece incompatability with
valid code. As such enabling them as a clang extension seems like a
reasonable approach.
Fixes#55124
Reviewed By: aaron.ballman
Differential Revision: https://reviews.llvm.org/D124487
0d6b574b87