Move HLSL builtins into hlsl namespace

Move HLSL builtins into hlsl namespace

Should have done this from the start. Since all the injected AST types
are in the hlsl namespace we should also put the header-defined types
and functions in there too.

This updates the basic_types test to run once with the namespaced types
and once without, and adds using declarations or namespaces calls in
other tests.

Reviewed By: python3kgae

Differential Revision: https://reviews.llvm.org/D135973