Fix completely bogus types for some builtins:

 * In C++, never create a FunctionNoProtoType for a builtin (fixes C++1z
   crasher from r289754).

 * Fix type of __sync_synchronize to be a no-parameter function rather than a
   varargs function. This matches GCC.

 * Fix type of vfprintf to match its actual type. We gave it a wrong type due
   to PR4290 (apparently autoconf generates invalid code and expects compilers
   to choke it down or it miscompiles the program; the relevant error in clang
   was downgraded to a warning in r122744 to fix other occurrences of this
   autoconf brokenness, so we don't need this workaround any more).

 * Turn off vararg argument checking for __noop, since it's not *really* a
   varargs function. Alternatively we could add custom type checking for it
   and synthesize parameter types matching the actual arguments in each call,
   but that seemed like overkill.

llvm-svn: 290146