[ConstraintSystem] Only use the new disjunction ordering under -swift-version 5.

This will minimize the chance of breaking code. Currently SwiftLint
has one "too complex" expression with this change. Further changes to
the solver may improve that situation, and potentially allow us to
move this out from -swift-version 5 if we're willing to take the risk
of breaking some code as a result.

Author: rudkx

lib/Sema/CSSolver.cpp

@@ -1577,8 +1577,9 @@ Constraint *ConstraintSystem::selectDisjunction() {
   if (auto *disjunction = selectBestBindingDisjunction(*this, disjunctions))
     return disjunction;
 
-  if (auto *disjunction = selectApplyDisjunction())
-    return disjunction;
+  if (getASTContext().isSwiftVersionAtLeast(5))
+    if (auto *disjunction = selectApplyDisjunction())
+      return disjunction;
 
   // Pick the disjunction with the smallest number of active choices.
   auto minDisjunction =

validation-test/Sema/type_checker_perf/fast/rdar33688063.swift

@@ -1,4 +1,4 @@
-// RUN: %target-typecheck-verify-swift -solver-expression-time-threshold=1 -swift-version 4
+// RUN: %target-typecheck-verify-swift -solver-expression-time-threshold=1 -swift-version 5
 // REQUIRES: tools-release,no_asserts
 
 let _ = 1 | UInt32(0) << 0 | UInt32(1) << 1 | UInt32(2) << 2 | UInt32(3) << 3 | UInt32(4) << 4

validation-test/Sema/type_checker_perf/slow/expression_too_complex_4.swift

@@ -1,4 +1,4 @@
-// RUN: %target-typecheck-verify-swift -swift-version 4 -solver-expression-time-threshold=1
+// RUN: %target-typecheck-verify-swift -swift-version 4 -solver-expression-time-threshold=1 -swift-version 5
 // REQUIRES: tools-release,no_asserts
 
 func test(_ i: Int, _ j: Int) -> Int {

validation-test/Sema/type_checker_perf/slow/rdar22022980.swift

@@ -1,4 +1,4 @@
-// RUN: %target-typecheck-verify-swift -solver-expression-time-threshold=1
+// RUN: %target-typecheck-verify-swift -solver-expression-time-threshold=1 -swift-version 5
 // REQUIRES: tools-release,no_asserts
 
 _ = [1, 3, 5, 7, 11].filter{ $0 == 1 || $0 == 3 || $0 == 11 || $0 == 1 || $0 == 3 || $0 == 11 } == [ 1, 3, 11 ]