[c++20] P1143R2: Add support for the C++20 'constinit' keyword.

[c++20] P1143R2: Add support for the C++20 'constinit' keyword.

This is mostly the same as the
[[clang::require_constant_initialization]] attribute, but has a couple
of additional syntactic and semantic restrictions.

In passing, I added a warning for the attribute form being added after
we have already seen the initialization of the variable (but before we
see the definition); that case previously slipped between the cracks and
the attribute was silently ignored.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@370972 91177308-0d34-0410-b5e6-96231b3b80d8