Use lit.util.executeCommand instead of our own version

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@222717 91177308-0d34-0410-b5e6-96231b3b80d8
Merge branch 'master' of http://llvm.org/git/libcxx
Fixes to get libc++ building on sun solaris. Patch from C Bergstrom.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@222794 91177308-0d34-0410-b5e6-96231b3b80d8
Merge branch 'master' of http://llvm.org/git/libcxx