compiler: Don't crash on erroneous array return types.

Another issue with erroneous array types.  When an erroneous array
    type is in a function's signature, particularly the return type, we
    must guarantee that type is changed into an error type.  Otherwise,
    any operations that work on arrays and slices will crash when applied
    to the erroneous array return type.
    
    Fixes golang/go#12939.
    
    Reviewed-on: https://go-review.googlesource.com/16235

From-SVN: r232858

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-721c778adb8f99d8a6b7795dbad86013ccc9ba91
+9e68d67d65fd72b9b4f163f2f26e15cd0d3e2cd2
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/parse.cc

@@ -419,6 +419,8 @@ Parse::array_type(bool may_use_ellipsis)
     }
 
   Type* element_type = this->type();
+  if (element_type->is_error_type())
+    return Type::make_error_type();
 
   return Type::make_array_type(element_type, length);
 }