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
Driver: Handle /GR- in a compatible way with MSVC

There are slight differences between /GR- and -fno-rtti which made
mapping one to the other inappropriate.

-fno-rtti disables dynamic_cast, typeid, and does not emit RTTI related
information for the v-table.

/GR- does not generate complete object locators and thus will not
reference them in vftables.  However, constructs like dynamic_cast and
typeid are permitted.

This should bring our implementation of RTTI up to semantic parity with
MSVC modulo bugs.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212138 91177308-0d34-0410-b5e6-96231b3b80d8
CodeGenCXX: Add tests for RTTI in VFTables

This test was supposed to go in with r212125.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212139 91177308-0d34-0410-b5e6-96231b3b80d8
CodeGen: Add some comments describing VFTable CG

No functionality change, just some comments to describe what is going
on.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212142 91177308-0d34-0410-b5e6-96231b3b80d8