[CSSimplify] CGFloat-Double: Fix ambiguity when assigning CGFloat to double property/variable

Situations like:
```
  let _: Double = <<CGFloat>>
  <var/property of type Double> = <<CGFloat>>
```

Used to be supported due to an incorrect fix added in
diagnostic mode. Keeping impact the same here means
that right-hand side of the assignment is allowed to
maintain CGFloat until the very end which minimizes
the number of conversions used.

Resolves: rdar://139675914

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -14671,6 +14671,19 @@ ConstraintSystem::simplifyRestrictedConstraintImpl(
         if (auto depth = getExprDepth(anchor))
           impact = (*depth + 1) * impact;
       }
+    } else if (locator.directlyAt<AssignExpr>() ||
+               locator.endsWith<LocatorPathElt::ContextualType>()) {
+      // Situations like:
+      //
+      //  let _: Double = <<CGFloat>>
+      //  <var/property of type Double> = <<CGFloat>>
+      //
+      // Used to be supported due to an incorrect fix added in
+      // diagnostic mode. Keeping impact the same here means
+      // that right-hand side of the assignment is allowed to
+      // maintain CGFloat until the very end which minimizes
+      // the number of conversions used.
+      impact = 0;
     }
 
     increaseScore(SK_ImplicitValueConversion, locator, impact);

validation-test/Sema/implicit_cgfloat_double_conversion_correctness.swift

@@ -0,0 +1,45 @@
+// RUN: %target-typecheck-verify-swift
+
+// REQUIRES: objc_interop
+
+// Note this cannot use a fake Foundation because it lacks required operator overloads
+
+import Foundation
+import CoreGraphics
+
+do {
+  func test(a: CGFloat, b: CGFloat) {
+    let _: Double = a + b + 1 // Ok
+    let _: Double = a + b + 1.0 // Ok
+
+    var test: Double
+
+    test = a + b + 1 // Ok
+    test = a + b + 1.0 // Ok
+
+    _ = test
+
+    let _ = a + b + 1 // Ok
+    let _ = a + b + 1.0 // Ok
+  }
+}
+
+func test(cond: Bool, a: CGFloat, b: CGFloat) {
+  var test: Double
+
+  let width = a - b // CGFloat
+
+  if cond {
+    test = (width - 10) / 2 // Ok
+  } else {
+    test = (width - 20.0) / 3 // Ok
+  }
+
+  _ = test
+}
+
+func test_atan_ambiguity(points: (CGPoint, CGPoint)) {
+  var test = 0.0
+  test = atan((points.1.y - points.0.y) / (points.1.x - points.0.x)) // Ok
+  _ = test
+}