Win64: Disable uwtable until we fix .pdata emission in LLVM

LLVM r211399 started emitting .pdata for win64 by default.
Unfortunately, it produces invalid object files.  I plan to fix that
Soon.  For now, don't request unwind tables.  This fixes the clang-cl
self-host on win64.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212137 91177308-0d34-0410-b5e6-96231b3b80d8