[HLSL] Adjust access specifier behavior

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