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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@41521 91177308-0d34-0410-b5e6-96231b3b80d8