Fix a bug in FileCheck that wouldn't let define variables as follows:

; CHECK: [[VAR:[a-z]]]

The problem was that to find the end of the regex var definition, it was
simplistically looking for the next ]] and finding the incorrect one. A
better approach is to count nesting of brackets (taking escaping into
account). This way the brackets that are part of the regex can be discovered
and skipped properly, and the ]] ending is detected in the right place.

llvm-svn: 169109