Rework Parser-Sema interface for Objective-C @catch exception object

Rework Parser-Sema interface for Objective-C @catch exception object
arguments. Rather than having the parser call ActOnParamDeclarator
(which is a bit of a hack), call a new ActOnObjCExceptionDecl
action. We'll be moving more functionality into this handler to
perform earlier checking of @catch.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@102222 91177308-0d34-0410-b5e6-96231b3b80d8