[lit] Eliminate parent argument from TestingConfig.frompath(), which is effectively unused.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188032 91177308-0d34-0410-b5e6-96231b3b80d8
[lit] Only create config copies when a local config file is present.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188033 91177308-0d34-0410-b5e6-96231b3b80d8
[lit] Eliminate mustExist parameter from TestingConfig.frompath().

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188034 91177308-0d34-0410-b5e6-96231b3b80d8