-Introduce CheckerV2, a set of templates for convenient declaration & registration of checkers.
 Currently useful just for checkers working on the AST not the path-sensitive ones.
-Enhance CheckerManager to actually collect the checkers and turn it into the entry point for
 running the checkers.
-Use the new mechanism for the LLVMConventionsChecker.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@125778 91177308-0d34-0410-b5e6-96231b3b80d8