Tweak diagnostic wording for init list narrowing

The conventional form is '<action> to silence this warning'.

Also call the diagnostic an 'issue' rather than a 'message' because the latter
term is more widely used with reference to message expressions.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209052 91177308-0d34-0410-b5e6-96231b3b80d8