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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197378 91177308-0d34-0410-b5e6-96231b3b80d8