test: pass correct objcopy and ar paths to the test runner

Summary:
The test runner has code to autodetect this, but it's not very smart --
in particular, it fails in the case where we build the test executables
with the just-built clang. Since cmake already has the knowledge about
the right toolchain, we can just have it pass the appropriate flags to
the test runner.

This also removes the "temporary" cache-scrubbing hack added a couple
months ago.

Reviewers: zturner, beanz

Subscribers: mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D30453

llvm-svn: 296593