Fully specify the mangling of non-type template arguments.

This includes the material from itanium-cxx-abi#47 (non-type template
arguments), itanium-cxx-abi#63 (class constants), and the template-param-decl
portions of itanium-cxx-abi#85 (C++20 lambda-expressions).

@zygoloid gets credit for most of this, although I've made a few
substantive changes from his suggestions.