Small improvements to createOutputFile.

* Use a single stat to find out if the file exists and if it is a regular file.
* Use early returns when possible.
* Add comments explaining why we have each check.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185091 91177308-0d34-0410-b5e6-96231b3b80d8