Summary:
This new alias takes a comma separated list of prefixes which allows
'--check-prefix=A --check-prefix=B --check-prefix=C' to be written as
'--check-prefixes=A,B,C'.
Reviewers: probinson
Subscribers: probinson, llvm-commits, dsanders
Differential Revision: http://reviews.llvm.org/D21293
llvm-svn: 272670