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