Make clang behave in a gcc-compatible way in the presence of multiple flags...

Make clang behave in a gcc-compatible way in the presence of multiple flags for the same x86 target feature (e.g. -mno-sse -msse).  gcc uses a somewhat unintuitive algorithm here in that the enabled SSE instructions is based on the order of the *last* flag for *each* feature-level, so that "-mno-sse -msse2" only enables SSE2, but "-mno-sse -msse2 -msse" enables all SSE levels.

Issue reported on cfe-dev.

llvm-svn: 134296