PDL: Remove Expr ast nodes

Expr ast nodes were initially introduced with the mind they
would be used to describe generic values and size modifiers,
they are only used for constraint values at the moment.

Remove the Expr in favor of optional value, tag_id fields in
constraints. This makes the ast more uniform.

Test: parse and lint test/*.pdl files
(cherry picked from https://android-review.googlesource.com/q/commit:435f630f56179eeabbbfad6377d59b39eb85f76c)
Merged-In: I0528d2567e0bda18e7d9fa1282015ebaff053577
Change-Id: I0528d2567e0bda18e7d9fa1282015ebaff053577