lit: execfile() isn't present in Python 3.3

On the other hand, exec(compile()) doesn't work in older Python versions in the
2.x series.

This commit introduces exec(compile()) with a fallback to plain exec(). That'll
hopefully hit the sweet spot in terms of version support.

Followup to r198766 which added enhanced source locations for lit cfg parsing.

llvm-svn: 199006