Revert "glcpp: Demote "macro redefined" from an error to a warning"

This reverts commit d3df641f0a.

The original commit had sat unpushed on my machine for months. By the
time I found it again, I had forgotten that we had decided not to use
this change after all, (the relevant test was removed long ago).

src/glsl/glcpp/glcpp-parse.y

@@ -1687,8 +1687,8 @@ _define_object_macro (glcpp_parser_t *parser,
 			talloc_free (macro);
 			return;
 		}
-		glcpp_warning (loc, parser, "Redefinition of macro %s\n",
-			       identifier);
+		glcpp_error (loc, parser, "Redefinition of macro %s\n",
+			     identifier);
 	}
 
 	hash_table_insert (parser->defines, macro, identifier);
@@ -1718,8 +1718,8 @@ _define_function_macro (glcpp_parser_t *parser,
 			talloc_free (macro);
 			return;
 		}
-		glcpp_warning (loc, parser, "Redefinition of macro %s\n",
-			       identifier);
+		glcpp_error (loc, parser, "Redefinition of macro %s\n",
+			     identifier);
 	}
 
 	hash_table_insert (parser->defines, macro, identifier);