Revert "Merge pull request #2957 from paulp/pr/parser-improvements"

This reverts commit 884e1ce762d98b29594146d37b85384581d9ba96, reversing
changes made to f6fcc4431f272c707d49de68add532c452dd4b0f.
Add position check for regression introduced by #2957

The starting bound for ValDefs in #2957 is distinct from the expected result, e.g.

[4:9]val x = [8:9]0
instead of
[0:9]val x = [8:9]0
Merge pull request #2980 from huitseeker/revert-2957

Revert #2957