Implement more thoughful error recovery when dealing with bogus declarator types.

For example, the following code was resulting in spurious warnings. This was the result of
Sema::GetTypeForDeclarator() synthesizing a type to hand back to the caller (in this case,
"int []", instead of "struct s[]", which is invalid).

struct s; 
struct s* t (struct s z[]) {   // expected-error {{array has incomplete element type}}
  return z;
}

Strategy: Flag the error in Declarator/DeclaratorChunk. This info is later stored in 
the ParmVarDecl. If the decl is referenced, Sema::ParseIdentifierExpr() will check if 
the type is invalid. If so, it quietly returns "true", without instantiating a DeclRefExpr.

This seems to work nicely. If Chris is happy with the approach, I will generalize this to
all VarDecls.

llvm-svn: 41521