[SemaCXX] Add diagnostics to require_constant_initialization

Summary:
This hooks up the detailed diagnostics of why constant initialization was
not possible if require_constant_initialization reports an error.
I have updated the test to account for the new notes.

Reviewed By: EricWF
Subscribers: cfe-commits
Differential Revision: https://reviews.llvm.org/D24371

llvm-svn: 304451