[c++1z] Most of N4268 (allow constant evaluation for non-type template arguments).

We don't yet support pointer-to-member template arguments that have undergone
pointer-to-member conversions, mostly because we don't have a mangling for them yet.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@222807 91177308-0d34-0410-b5e6-96231b3b80d8