[NFC][libcxx] Print human-friendly command line when lit test fails

We used to print a Python list corresponding to the command. It is more
useful to print the joined string so it can be copy/pasted directly when
a test fails.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@347471 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed