Fix parse tree of DROP TRANSFORM and COMMENT ON TRANSFORM

 Fix parse tree of DROP TRANSFORM and COMMENT ON TRANSFORM
 
 The plain C string language name needs to be wrapped in makeString() so
 that the parse tree is copyable.  This is detectable by
 -DCOPY_PARSE_PLAN_TREES.  Add a test case for the COMMENT case.
 
 Also make the quoting in the error messages more consistent.
 
 discovered by Tom Lane