Use strtof and strtod for floats and doubles respectively instead of
always using strtold. The other parts of the change are already implemented
in libc++.
This patch also has a drive by fix to wbuffer_convert::underflow() which
prevents it from calling memmove(buff, null, 0).
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@273106 91177308-0d34-0410-b5e6-96231b3b80d8
