[analyzer] change "tag" in ProgramPoint from "void*" to a ProgramPointTag*.

Having a notion of an actual ProgramPointTag will aid in introspection of the analyzer's behavior.
For example, the GraphViz output of the analyzer will pretty-print the tags in a useful manner.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@137529 91177308-0d34-0410-b5e6-96231b3b80d8
16 files changed