Reland "Use Clang's __has_* macros in Compiler.h ..." with fixes

This reverts r185841 and relands r185831 without using
__has_attribute(const).

Clang prior to r161767 (between 3.1 and 3.2) does not accept
__has_attribute(const) due to rdar://10253857.  __const and __const__
are both keyword aliases of const, so they don't work either.

I was able to repro the buildbot failure using clang 3.1 and this patch
fixes it.  Various important versions of XCode use clang 2.9-ish, so
this workaround is necessary.