[CSRanking] Detect cases where overload choices are incomparable

If constraint system is underconstrained e.g. because there are
editor placeholders, it's possible to end up with multiple solutions
where each ambiguous declaration is going to have its own overload kind:

```swift
func foo(_: Int) -> [Int] { ... }
func foo(_: Double) -> (result: String, count: Int) { ... }

_ = foo(<#arg#>).count
```

In this case solver would produce 2 solutions: one where `count`
is a property reference on `[Int]` and another one is tuple access
for a `count:` element.

Resolves: rdar://problem/49712598

Author: xedin

lib/Sema/CSRanking.cpp

@@ -799,6 +799,21 @@ SolutionCompareResult ConstraintSystem::compareSolutions(
     if (sameOverloadChoice(choice1, choice2))
       continue;
 
+    // If constraint system is underconstrained e.g. because there are
+    // editor placeholders, it's possible to end up with multiple solutions
+    // where each ambiguous declaration is going to have its own overload kind:
+    //
+    // func foo(_: Int) -> [Int] { ... }
+    // func foo(_: Double) -> (result: String, count: Int) { ... }
+    //
+    // _ = foo(<#arg#>).count
+    //
+    // In this case solver would produce 2 solutions: one where `count`
+    // is a property reference on `[Int]` and another one is tuple access
+    // for a `count:` element.
+    if (choice1.isDecl() != choice2.isDecl())
+      return SolutionCompareResult::Incomparable;
+
     auto decl1 = choice1.getDecl();
     auto dc1 = decl1->getDeclContext();
     auto decl2 = choice2.getDecl();

test/Sema/editor_placeholders.swift

@@ -29,3 +29,10 @@ f(<#T##String#>) // expected-error{{editor placeholder in source file}} expected
 for x in <#T#> { // expected-error{{editor placeholder in source file}} expected-error{{for-in loop requires '()' to conform to 'Sequence'}}
 
 }
+
+// rdar://problem/49712598 - crash while trying to rank solutions with different kinds of overloads
+func test_ambiguity_with_placeholders(pairs: [(rank: Int, count: Int)]) -> Bool {
+  return pairs[<#^ARG^#>].count == 2
+  // expected-error@-1 {{editor placeholder in source file}}
+  // expected-error@-2 {{ambiguous use of 'subscript(_:)'}}
+}