Introduce a fix to detect and diagnose situations when omitted
generic arguments couldn't be deduced by the solver based on
the enclosing context.
Example:
```swift
struct S<T> {
}
_ = S() // There is not enough context to deduce `T`
```
Resolves: rdar://problem/51203824
All of the open existentials should be removed, along with their
opaque value expressions, after sub-expression type-check. Because
diagnostics might pick next sub-expression from constraint and its
anchor could point to sub-expression which has only opaque value
without enclosing open existential, which is going to trip up sanitizer.
Resolves: rdar://problem/46544601
