raw_stream formatter: [Win32] Use std::signbit() if available, instead of _fpclass().

FIXME: It should be generic to C++11. For now, it is dedicated to mingw-w64.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199052 91177308-0d34-0410-b5e6-96231b3b80d8