[flang] Dump parse tree even if semantic error occurs

When -fdebug-dump-parse-tree is specified, dump the parse tree even
if semantic errors have occurred. This is necessary to see the parse
tree after it has been transformed.

Original-commit: flang-compiler/f18@bbdb0717da953c959cb1b6c531684952c239b98b
Reviewed-on: https://github.com/flang-compiler/f18/pull/417