[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/ConstraintSystem.cpp

@@ -100,13 +100,9 @@ void ConstraintSystem::incrementLeafScopes() {
 
 bool ConstraintSystem::hasFreeTypeVariables() {
   // Look for any free type variables.
-  for (auto tv : TypeVariables) {
-    if (!tv->getImpl().hasRepresentativeOrFixed()) {
-      return true;
-    }
-  }
-  
-  return false;
+  return llvm::any_of(TypeVariables, [](const TypeVariableType *typeVar) {
+    return !typeVar->getImpl().hasRepresentativeOrFixed();
+  });
 }
 
 void ConstraintSystem::addTypeVariable(TypeVariableType *typeVar) {