[c++20] For P0732R2 / P1907R1: Basic code generation and name

mangling support for non-type template parameters of class type and
template parameter objects.

The Itanium side of this follows the approach I proposed in
https://github.com/itanium-cxx-abi/cxx-abi/issues/47 on 2020-09-06.

The MSVC side of this was determined empirically by observing MSVC's
output.

Differential Revision: https://reviews.llvm.org/D89998