Introduce __has_extension macro

__has_extension is a function-like macro which takes the same set
of feature identifiers as __has_feature.  It evaluates to 1 if the
feature is supported by Clang in the current language (either as a
language extension or a standard language feature) or 0 if not.

At the same time, add support for the C1X feature identifiers
c_generic_selections (renamed from generic_selections) and
c_static_assert, and document them.

Patch by myself and Jean-Daniel Dupas.

llvm-svn: 131308