[ThinLTO] Enable index-only WPD from clang

[ThinLTO] Enable index-only WPD from clang

Summary:
To trigger the index-only Whole Program Devirt support added to LLVM, we
need to be able to specify -fno-split-lto-unit in conjunction with
-fwhole-program-vtables. Keep the default for -fwhole-program-vtables as
-fsplit-lto-unit, but don't error on that option combination.

Reviewers: pcc

Subscribers: mehdi_amini, inglorion, steven_wu, dexonsmith, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68029

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@373370 91177308-0d34-0410-b5e6-96231b3b80d8