lit: Provide file location in cfg error messages

Python doesn't do a good job at diagnosing string exec() so use execfile()
where available.

This should be a timesaver when trying to get to the bottom of build bot
failures.

Before:

    File "llvm/utils/lit/lit/TestingConfig.py", line 93, in load_from_path
      exec("exec data in cfg_globals")
    File "<string>", line 1, in <module>
    File "<string>", line 194, in <module>
  NameError: name 'typo' is not defined

After:

    File "llvm/utils/lit/lit/TestingConfig.py", line 95, in load_from_path
    execfile(path, cfg_globals)
    File "clang/test/lit.cfg", line 194, in <module>
      typo
      ^~~~
  NameError: name 'typo' is not defined