[libcxx] Allow the use of ccache when running the test suite.

Summary:
In order to get the bots running quicker I would like to be able to use ccache
with the test suite. This patch adds support for running the test suite using
ccache. To use ccache pass `--param=use_ccache=true` when running the test suite.

ccache will not cache any command that invokes ld, so the build step needs to be
split into two separate compile commands. The cost of splitting the build step
into two parts when not using ccache seems to be minimal. On my machine I saw a
difference of ~5 seconds on a 5 minute test suite run.

A full test suite run with ccache generates about 250MB of cached data.

I recorded the following times for running the test suite in the following configurations:
- no ccache: 340s
- initial ccache run: 380s
- rerun with ccache (no changes): 53s.
- rerun with ccache (<string> changed): 80s
- rerun with ccache (<cmath> changed): 169s
- rerun with ccache (<valarray> changed): 69s





Reviewers: mclow.lists, jroelofs, danalbert

Reviewed By: jroelofs

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D6647