Fix bug with check for missing remote equations

[krame505]

Changes

This fixes an issue with chaining multiple levels of decoration through references across multiple productions:

production foo
top::Expr ::= e::Expr
{
  top.errors = ... e.env ...;
  forwards to bar(@e);
}

production bar
top::Expr ::= e::Expr
{
  forwards to baz(@e);
}

production baz
top::Expr ::= e::Expr
{
  e.env = top.env;
}

foo depends on e.env, but this equation is in baz, not bar. We need to find all decoration sites for e, and ensure that there is an env equation at any of them.

Documentation

Added source comments and a test case.

[krame505]

I discovered that there is a related issue in the translation for this scenario that requires a bit of refactoring - will be another PR shortly.