[HLSL] Support register binding attribute on global variable

Allow register binding attribute on variables.

Report warning when register binding attribute applies to local variable or static variable.
It will be ignored in this case.

Type check for register binding is tracked with https://github.com/llvm/llvm-project/issues/57886.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D134617