fix: Support sum variants with sum types containing type variables in constant folding

[maximilianruesch]

Addresses constant folding support for sum variants where sum types contain type variables by:

• Loading the value via LoadConstant if possible (i.e. if not parametrized)
• Otherwise, loading the value via a Tag op if possible (i.e. if it is a sum variant)
  • To reduce the work done here, only sum variants with no child values are supported (others are skipped), so we do not have to recursively reconstruct the nodes required for the values.
• Otherwise, doing nothing

Based on #1580

Closes #1536