implement support for propagating *features* down to the code generator

and defining target-specific macros based on them (like __SSE3__ and 
friends).  After extensive discussion with Daniel, this work will need
driver support, which will translate things like -msse3 into a -mattr 
feature.  Until this work is done, the code in clang.cpp is disabled and
the X86TargetInfo ctor still defaults to SSE2.  With these two things
changed, this code will work.  PR3634

llvm-svn: 65966