start converting over to attr(overloadable). Unfortunately, this

produces really horrible diagnostics when overload ambiguities
happen:

t.c:10:10: error: call to '__tg_acos' is ambiguous; candidates are:
  return acos(x);
         ^~~~
In file included from t.c:1:
/Users/sabre/llvm/Debug/Headers/tgmath-sofar.h:49:1: note: candidate function
__TG_RC_1(x, acos, cacos)
^
/Users/sabre/llvm/Debug/Headers/tgmath-sofar.h:49:1: note: candidate function
__TG_RC_1(x, acos, cacos)
^
/Users/sabre/llvm/Debug/Headers/tgmath-sofar.h:49:1: note: candidate function
__TG_RC_1(x, acos, cacos)
^
/Users/sabre/llvm/Debug/Headers/tgmath-sofar.h:49:1: note: candidate function
__TG_RC_1(x, acos, cacos)
^
/Users/sabre/llvm/Debug/Headers/tgmath-sofar.h:49:1: note: candidate function
__TG_RC_1(x, acos, cacos)
^
/Users/sabre/llvm/Debug/Headers/tgmath-sofar.h:49:1: note: candidate function
__TG_RC_1(x, acos, cacos)
^

A possible fix is to just not use macros for this, which I'll probably go for,
but it would be nice to emit the type at the call, so we know what we asked for!

llvm-svn: 64720