Recommit: 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
optimization failures that were added to llvm to produce a warning when
loop vectorization is explicitly specified (using a pragma clang loop directive)
but fails.

Update renames warning flag name to avoid indicating the flag's severity and
adds a test.

Reviewed by Alp Toker