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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198766 91177308-0d34-0410-b5e6-96231b3b80d8

utils/lit/lit/TestingConfig.py

@@ -1,7 +1,7 @@
 import os
 import sys
 
-PY2 = sys.version_info[0] < 3
+OldPy = sys.version_info[0] == 2 and sys.version_info[1] < 6
 
 class TestingConfig:
     """"
@@ -74,6 +74,8 @@ class TestingConfig:
         """
 
         # Load the config script data.
+        data = None
+        if OldPy:
             f = open(path)
             try:
                 data = f.read()
@@ -87,10 +89,10 @@ class TestingConfig:
         cfg_globals['lit_config'] = litConfig
         cfg_globals['__file__'] = path
         try:
-            if PY2:
+            if OldPy:
                 exec("exec data in cfg_globals")
             else:
-                exec(data, cfg_globals)
+                execfile(path, cfg_globals)
             if litConfig.debug:
                 litConfig.note('... loaded config %r' % path)
         except SystemExit: