Refactor HLSLExternalSemaSource's builder API

[bogner]

The BuiltinTypeDeclBuilder in HLSLExternalSemaSource is used to simplify building the various HLSL builtin types (like resources), but it turns out it's a bit clunky for some of the common patterns. Refactor this to make it easier to read and write the definitions of things like RWBuffer.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Justin Bogner (bogner)

The BuiltinTypeDeclBuilder in HLSLExternalSemaSource is used to simplify building the various HLSL builtin types (like resources), but it turns out it's a bit clunky for some of the common patterns. Refactor this to make it easier to read and write the definitions of things like RWBuffer.