Other type substitutors (classic, cone-based) respect the type
annotations and copy them to the resulting type for all
cases, including type parameters.
Without this change we cannot correctly match expect/actuals
when we replace actual type parameters with expect ones in case
the actual type parameter has type enhancement annotations
(e.g., `@FlexibleNullability`).
We transfer all annotations (and not conservatively copy only type
enhancement annotations), as 1) other substitutors do that 2) other
IR type substitution utilities (e.g., `IrType.substitute`) do that.
As we will attempt reimplementing all IR type substitution utilities
via IrTypeSubstitutor, it also makes sense to completely align
the behavior.
marat.akhin
a2dda9e25b