Teaching lit to expand glob expressions

Hi all, in https://reviews.llvm.org/D30380 I’ve uploaded a patch that allows lit RUN lines to expand expressions that will be globbed against the file system.

It already has an LGTM, but given that lit is so fundamental and this adds core functionality, I thought I’d post it here in case anyone else has any thoughts. I plan to submit this tomorrow if there are no objections or concerns.