[clang-tidy] The patch extends the existing command line option -check-suffix

(with alias -check-suffixes) to accept multiple comma-separated FileCheck prefixes.

Usage:

// RUN: %check_clang_tidy -check-suffix=USING-C,USING-D %s misc-unused-using-decls %t -- -- ...
or for the same:
// RUN: %check_clang_tidy -check-suffixes=USING-C,USING-D %s misc-unused-using-decls %t -- -- ...

Differential Revision: https://reviews.llvm.org/D52971

llvm-svn: 344015