Makefile.rules: Use an absolute path to the module cache directory.
Makefile.rules: Use an absolute path to the module cache directory.

This change is NFC, but it makes it more obvious in log files what happened.

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@340286 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-lldb,llvm-mirror/lldb,llvm-mirror/lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,llvm-mirror/lldb