[Diagnostics] Diagnose missing arguments in relation to contextual type

Function type has fewer arguments than expected by context:

```swift
func foo() {}
let _: (Int) -> Void = foo // expected 1 argument, got 0
```

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -3508,6 +3508,20 @@ bool AllowTypeOrInstanceMemberFailure::diagnoseAsError() {
     return true;
   }
 
+  // Function type has fewer arguments than expected by context:
+  //
+  // ```
+  // func foo() {}
+  // let _: (Int) -> Void = foo
+  // ```
+  if (locator->isLastElement(ConstraintLocator::ContextualType)) {
+    auto &cs = getConstraintSystem();
+    emitDiagnostic(anchor->getLoc(), diag::cannot_convert_initializer_value,
+                   getType(anchor), resolveType(cs.getContextualType()));
+    // TODO: It would be great so somehow point out which arguments are missing.
+    return true;
+  }
+
   return false;
 }