compiler: Make empty interface types for vars during parse time.

When making the type for a variable with an empty interface type,
the parser makes an interface type with a NULL method set and relies
on later passes to correct this.

For sink variables, which are ignored in later passes, the interface
method table is never finalized and a compile time assertion is issued.
Instead, the initial type generated by the parser should be the empty
interface type.

Fixes golang/go#11579.

Change-Id: I479559f270ddf88afc7a33103c0f56afda195d94
Reviewed-on: https://go-review.googlesource.com/12049
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: Chris Manghane

go/parse.cc

@@ -1225,7 +1225,11 @@ Parse::interface_type(bool record)
       methods = NULL;
     }
 
-  Interface_type* ret = Type::make_interface_type(methods, location);
+  Interface_type* ret;
+  if (methods == NULL)
+    ret = Type::make_empty_interface_type(location);
+  else
+    ret = Type::make_interface_type(methods, location);
   if (record)
     this->gogo_->record_interface_type(ret);
   return ret;