[FileCheck] Add --check-prefixes as a shorthand for multiple --check-prefix options.

[FileCheck] Add --check-prefixes as a shorthand for multiple --check-prefix options.

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@272670 91177308-0d34-0410-b5e6-96231b3b80d8