Revert "Clang support for -flto=thin." (bot failures)

Revert "Clang support for -flto=thin." (bot failures)

Rolling this back for now since there are a couple of bot failures on
the new tests I added, and I won't have a chance to look at them in detail
until later this afternoon. I think the new tests need some restrictions on
having the gold plugin available.

This reverts commit r250398.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@250402 91177308-0d34-0410-b5e6-96231b3b80d8