Added -iregex for case-insensitive regex to filter file names.

Summary:
-regex and -iregex both mimic options of the find utility.
Made the default list of extensions case-insensitive, so that it's not only C
and CPP extensions are accepted in upper case.

Reviewers: djasper

Reviewed By: djasper

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2415

llvm-svn: 197378