[X86] Rename some frame lowering variables

Old names, new names, and what they really mean:

- IsWin64 -> IsWin64CC: This is true on non-Windows x86_64 platforms
  when the ms_abi calling convention is used.
- IsWinEH -> IsWin64Prologue: True when the target is Win64, regardless
  of calling convention. Changes the prologue to obey the constraints of
  the Win64 unwinder.
- NeedsWinEH -> NeedsWinCFI: We're using the win64 prologue *and* the we
  want .xdata unwind tables. Analogous to NeedsDwarfCFI.

NFC

llvm-svn: 239836