[ConstraintSystem] Decouple designated types feature from Swift version

`selectApplyDisjunction` only makes sense in conjunction with
designated types feature because it's going to prioritize disjunctions
with arguments which are known to conform to literal protocols.

Prioritization like that is harmful without designated types feature
because it doesn't always lead to better constraint system splits,
and could prioritize bigger disjunctions which harms chances to
short-circuit solving.

Resolves: rdar://problem/47492691

Author: xedin

lib/Sema/CSSolver.cpp

@@ -1897,10 +1897,10 @@ Constraint *ConstraintSystem::selectDisjunction() {
   // disjunctions that we may not be able to short-circuit, allowing
   // us to eliminate behavior that is exponential in the number of
   // operators in the expression.
-  if (getASTContext().isSwiftVersionAtLeast(5) ||
-      TC.getLangOpts().SolverEnableOperatorDesignatedTypes)
+  if (TC.getLangOpts().SolverEnableOperatorDesignatedTypes) {
     if (auto *disjunction = selectApplyDisjunction())
       return disjunction;
+  }
 
   if (auto *disjunction = selectBestBindingDisjunction(*this, disjunctions))
     return disjunction;

validation-test/Sema/type_checker_perf/fast/rdar47492691.swift

@@ -0,0 +1,9 @@
+// RUN: %target-typecheck-verify-swift
+// REQUIRES: objc_interop
+
+import CoreGraphics
+import simd
+
+func test(foo: CGFloat, bar: CGFloat) {
+  _ = CGRect(x: 0.0 + 1.0, y: 0.0 + foo, width: 3.0 - 1 - 1 - 1.0, height: bar)
+}