[libcxx] Always convert 'use_system_cxx_lib' to an absolute path

Otherwise, some tests would fail when a relative path was passed,
because they'd use the relative path from a different directory
than the current working directory.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@348525 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed