[asan_symbolize] Workaround bug in old Python 2 versions.

The change landed in r358657 broke some of the buildbots because they
use an older version of Python 2 that raises this error.

```
File "/Volumes/data/dev/llvm/upstream/master/src/projects/compiler-rt/lib/asan/scripts/asan_symbolize.py", line 509
  exec(f.read(), globals_space, None)
SyntaxError: unqualified exec is not allowed in function 'load_plugin_from_file' it contains a nested function with free variables
```

I can reproduce this problem when using Python 2.7.6.

To workaround this some indirection has been added to prevent the broken
(the line at fault would never be executed) SyntaxError error in old
Python versions from being raised.

rdar://problem/49476995

llvm-svn: 358682