Change remaining references to lit.util.capture to use subprocess.check_output.

Summary:
The capture() function was removed in r306625. This should fix PGO breakages
reported by Michael Zolotukhin.

Reviewers: mzolotukhin

Subscribers: sanjoy, llvm-commits

Differential Revision: https://reviews.llvm.org/D35088

llvm-svn: 307320