SEH: Track users of __try so we can pick a per-func EH personality

There are four major kinds of declarations that cause code generation:
- FunctionDecl (includes CXXMethodDecl etc)
- ObjCMethodDecl
- BlockDecl
- CapturedDecl

This patch tracks __try usage on FunctionDecls and diagnoses __try usage
in other decls. If someone wants to use __try from ObjC, they can use it
from a free function, since the ObjC code will need an ObjC-style EH
personality.

Eventually we will want to look through CapturedDecls and track SEH
usage on the parent FunctionDecl, if present.

llvm-svn: 228058