Improve diagnostic on default-initializing const variables (PR20208).

This tweaks the diagnostic wording slighly, and adds a fixit on a note.
An alternative would be to add the fixit directly on the diagnostic, see
the review thread linked to from the bug for a few notes on that approach.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213725 91177308-0d34-0410-b5e6-96231b3b80d8