[HLSL] Add internal linkage attribute to resources
HLSL resources should not be externally visible from the module.
We made sure of this by marking them `static` as soon as they were declared.
However, this prevents us fixing issue #166458 because there is no way
to know if a resource has been explicitly marked `static`
by the user, and can therefore be assigned to.
This change is moves from making all resources `static` to adding Clang internal
linkage attribute to all non-static resource declarations as a global scope.
Existing tests verify that there is no change in how the resource globals are
emitted: `internal global`.