[IR] Move support for dxil::TypedPointerType to LLVM core IR.

This allows the construct to be shared between different backends. However, it
still remains illegal to use TypedPointerType in LLVM IR--the type is intended
to remain an auxiliary type, not a real LLVM type. So no support is provided for
LLVM-C, nor bitcode, nor LLVM assembly (besides the bare minimum needed to make
Type->dump() work properly).

Reviewed By: beanz, nikic, aeubanks

Differential Revision: https://reviews.llvm.org/D130592