compiler: avoid crash on real declaration of type with existing method

This avoids a compiler crash on invalid code.
    
    Fixes https://gcc.gnu.org/PR90272
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/174377

From-SVN: r270658

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-b117b468264665cfe6ec2cf3affb48330a704fa7
+9476f6183791477dd9b883f51e2a46b224227735
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/gogo.cc

@@ -7816,7 +7816,8 @@ Type_declaration::define_methods(Named_type* nt)
        p != this->methods_.end();
        ++p)
     {
-      if (!(*p)->func_value()->is_sink())
+      if ((*p)->is_function_declaration()
+	  || !(*p)->func_value()->is_sink())
 	nt->add_existing_method(*p);
     }
 }