When parsing an out-of-line member function declaration, we must delay

access-control diagnostics which arise from the portion of the declarator
following the scope specifier, just in case access is granted by
friending the individual method.  This can also happen with in-line
member function declarations of class templates due to templated-scope
friend declarations.

We were really playing fast-and-loose before with this sort of thing,
and it turned out to work because *most* friend functions are in file
scope.  Making us delay regardless of context exposed several bugs with
how we were manipulating delay.  I ended up needing a concept of a
context that's independent of the declarations in which it appears,
and then I actually had to make some things save contexts correctly,
but delay should be much cleaner now.

I also encapsulated all the delayed-diagnostics machinery in a single
subobject of Sema;  this is a pattern we might want to consider rolling
out to other components of Sema.

llvm-svn: 125485