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

Description

[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