Add support for parsing method prototypes (and other stuff required by @interface).

Add support for parsing method prototypes (and other stuff required by @interface).

Still need to finish Parser::ParseObjCMethodDecl(). Before I do, I need to do a minor
refactoring of ParseDeclarationOrFunctionDefinition(), to disallow function definitions.
At the moment, @inteface allows function defs (which is incorrect).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@41275 91177308-0d34-0410-b5e6-96231b3b80d8