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.
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.
CodeGenCXX: Add tests for RTTI in VFTables

This test was supposed to go in with r212125.
[FastISel] Factor out stackmap intrinsic selection code into a dedicated helper method. NFCI.
Run clang-format on llvm-size.cpp to tidy things up. No functional changes.