When using an old version of glibc, a ::isinf(double) and ::isnan(double)
function is provided, rather than just the macro required by C and C++.
Displace this function using _LIBCPP_PREFERRED_OVERLOAD where possible.
The only remaining case where we should get the wrong return type is now
glibc + libc++ + a non-clang compiler.
llvm-svn: 331241