Fix some C++ error recovery problems in init declarator parsing

that I noticed working on other things.

Instead of emitting:

t2.cc:1:8: error: use of undeclared identifier 'g'
int x(*g);
       ^
t2.cc:1:10: error: expected ')'
int x(*g);
         ^
t2.cc:1:6: note: to match this '('
int x(*g);
     ^

We now only emit:

t2.cc:1:7: warning: type specifier missing, defaults to 'int'
int x(*g);
      ^


Note that the example in SemaCXX/nested-name-spec.cpp:f4 is still
not great, we now produce both of:

void f4(undef::C); // expected-error {{use of undeclared identifier 'undef'}} \
                      expected-error {{variable has incomplete type 'void'}}

The second diagnostic should be silenced by something getting marked invalid.
I don't plan to fix this though.

llvm-svn: 68919