[libcxx] Improve reporting when running the lit test suite

Summary:
Running the test suite with -a will now properly show all the executed
commands. The reports also include the environment under which the test
is being executed, which is helpful for reproducing issues.

Reviewers: EricWF

Subscribers: christof, dexonsmith, libcxx-commits

Differential Revision: https://reviews.llvm.org/D53215

llvm-svn: 344700