Driver: Stop forcing frame pointer usage on Windows

Previously, we would use a frame pointer by default on non-Linux OSs. On
Linux, any optimization flags imply -fomit-frame-pointer. XCore always
defaulted to -fomit-frame-pointer.

Now x86 Windows matches our behavior on Linux. All other ISAs supported
by Windows (ARM, x64) use xdata information, and frame pointers aren't
useful. Frame pointers are now off by default for such targets, but can
be forced via -fno-omit-frame-pointer and code using alloca().

In fact, on Win64 our frame-pointer prologue is not describable with
UNWIND_INFO. This change is a workaround to avoid using the broken FP
using prologue for most functions. This is PR22467.

llvm-svn: 228236