[CSOptimizer] Infer argument candidates from calls to Double and CGFloat constructors

Helps situations like `1 + {Double, CGFloat}(...)` by inferring
a type for the second operand of `+` based on a type being constructed.

Currently limited to Double and CGFloat only since we need to
support implicit `Double<->CGFloat` conversion.

Helps situations like `1 + CGFloat(...)` by inferring a type for
the second operand of `+` based on a type being constructed.
Currently limited to known integer, floating-point and CGFloat types
since we know how they are structured.

include/swift/Sema/ConstraintSystem.h

@@ -497,6 +497,10 @@ public:
   /// literal (represented by `ArrayExpr` and `DictionaryExpr` in AST).
   bool isCollectionLiteralType() const;
 
+  /// Determine whether this type variable represents a result type of a
+  /// function call.
+  bool isFunctionResult() const;
+
   /// Retrieve the representative of the equivalence class to which this
   /// type variable belongs.
   ///