[lit/libcxx] Fix a remaining reference to lit.util.capture() in custom libcxx/Darwin code.

Summary:
This reference to lit.util.capture is functionally identical to
subprocess.check_output, so this change switches to call the library routine
directly.

Reviewers: mzolotukhin, EricWF

Reviewed By: mzolotukhin

Subscribers: sanjoy, llvm-commits

Differential Revision: https://reviews.llvm.org/D34841

llvm-svn: 306755