__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
Peter Collingbourne