[HLSL][NFC] Refactor HLSLExternalSemaSource (#131032)

Moving builder classes into separate files
`HLSLBuiltinTypeDeclBuilder.cpp`/`.h`, changing a some
`HLSLExternalSemaSource` methods to private and removing unused methods.

This is a prep work before we start adding more builtin types and
methods, like textures, resource constructors or matrices. For example
constructors could make use of the `BuiltinTypeMethodBuilder`, but this
helper class was defined in `HLSLExternalSemaSource.cpp` after the
method that creates a constructor. Rather than reshuffling the code one
big source file I am moving the builders into a separate cpp & header
file and placing the helper classes declarations up top.

Currently the new header only exposes `BuiltinTypeDeclBuilder` to
`HLSLExternalSemaSource`. In the future but we might decide to expose
more helper classes as needed.