[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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@344700 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed