Handle diagnostic warnings in Frontend diagnostic handler.

Clang uses a diagnostic handler to grab diagnostic messages so it can print them
with the line of source code they refer to. This patch extends this to handle
diagnostic warnings that were added to llvm to produce a warning when
loop vectorization is explicitly specified (using a pragma clang loop directive)
but fails.

Reviewed by: Aaron Ballman


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213112 91177308-0d34-0410-b5e6-96231b3b80d8