[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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@306755 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed