[clang-tidy] Customize FileCheck prefix in check_clang-tidy.py

The patch introduces a new command line option '-check-suffix' for check_clang_tidy.py 
to allow multiple %check_clang_tidy% in a single test file.

Sample:

// RUN: %check_clang_tidy -check-suffix=FLAG-1 %s misc-unused-using-decls %t -- -- <options-set-1>
// RUN: %check_clang_tidy -check-suffix=FLAG-2 %s misc-unused-using-decls %t -- -- <options-set-2> 
...
+// CHECK-MESSAGES-FLAG-1: :[[@LINE-4]]:10: warning: using decl 'B' is unused [misc-unused-using-decls]
+// CHECK-MESSAGES-FLAG-2: :[[@LINE-7]]:10: warning: using decl 'A' is unused [misc-unused-using-decls]
+// CHECK-FIXES-FLAG-1-NOT: using a::A;$
+// CHECK-FIXES-FLAG-2-NOT: using a::B;$

Differential Revision: https://reviews.llvm.org/D45776

llvm-svn: 330511