[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

test/Constraints/tuple.swift

@@ -219,13 +219,13 @@ extension r25271859 {
   func map<U>(f: (T) -> U) -> r25271859<U> {
   }
 
-  func andThen<U>(f: (T) -> r25271859<U>) {
+  func andThen<U>(f: (T) -> r25271859<U>) { // expected-note {{in call to function 'andThen(f:)'}}
   }
 }
 
 func f(a : r25271859<(Float, Int)>) {
-  a.map { $0.0 }
-    .andThen { _ in   // expected-error {{unable to infer complex closure return type; add explicit type to disambiguate}} {{18-18=-> r25271859<String> }}
+  a.map { $0.0 } // expected-error {{generic parameter 'U' could not be inferred}} (This is related to how solver is setup with multiple statements)
+    .andThen { _ in
       print("hello") // comment this out and it runs, leave any form of print in and it doesn't
       return r25271859<String>()
   }