[ConstraintSystem] Detect and diagnose missing generic arguments

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

lib/Sema/CSSimplify.cpp

@@ -6659,6 +6659,7 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyFixConstraint(
   case FixKind::AllowAnyObjectKeyPathRoot:
   case FixKind::TreatKeyPathSubscriptIndexAsHashable:
   case FixKind::AllowInvalidRefInKeyPath:
+  case FixKind::ExplicitlySpecifyGenericArguments:
     llvm_unreachable("handled elsewhere");
   }