Add Sema support for __builtin_setjmp/__builtin_longjmp. The primary

reason for adding these is to error out in CodeGen when trying to generate
them instead of silently emitting a call to a non-existent function.

(Note that it is not valid to lower these to setjmp/longjmp; in addition 
to that lowering being different from the intent, setjmp and longjmp 
require a larger buffer.)

llvm-svn: 70658