Make '-Werror=frame-larger-than=' and associated diagnostic pragmas GCC-compatible

It turns out the trailing '=' really is part of the option name spelling and
treating it as such gets us compatible with GCC's -Werror= and pragmas.

(GCC doesn't appear to support any -Wno- form for this diagnostic but we do.)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210503 91177308-0d34-0410-b5e6-96231b3b80d8