[TypeChecker] Rework type-checking of `for-in` statements
Change the way how `for-in` statements are type-checked, where a lot of
information has to be recorded in a statement in order for SILGen to generate
appropriate calls to `makeIterator` and `next` witnesses.
Instead of just finding a witness, solver would now:
- Synthesize:
- Iterator variable with its initializer - `$generator = <sequence>.makeIterator()`;
- Call to `$generator.next()` which is called by each loop iteration;
- Verify that type of pattern is convertible to type of `next()`
Doing so significantly simplifies SILGen, removes the need to carry type information
in `ForEachStmt` and enables iterating over existential types.
Resolves: rdar://92177656
