[ConstraintSystem] Add a option to re-enable performance hacks

Instead of checking `EnableConstraintSolverPerformanceHacks`
directly, let's use an option which is easier to set i.e.
when a block list is implemented.

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -1964,6 +1964,9 @@ enum class ConstraintSystemFlags {
 
   /// Disable macro expansions.
   DisableMacroExpansions = 0x80,
+
+  /// Enable old type-checker performance hacks.
+  EnablePerformanceHacks = 0x100,
 };
 
 /// Options that affect the constraint system as a whole.
@@ -3604,10 +3607,9 @@ class ConstraintSystem {
   }
 
   /// Check whether old type-checker performance hacks has been explicitly
-  /// enabled by a flag.
+  /// enabled.
   bool performanceHacksEnabled() const {
-    return getASTContext()
-                .TypeCheckerOpts.EnableConstraintSolverPerformanceHacks;
+    return Options.contains(ConstraintSystemFlags::EnablePerformanceHacks);
   }
 
   /// Log and record the application of the fix. Return true iff any

lib/Sema/TypeCheckConstraints.cpp

@@ -466,6 +466,9 @@ TypeChecker::typeCheckTarget(SyntacticElementTarget &target,
   if (options.contains(TypeCheckExprFlags::DisableMacroExpansions))
     csOptions |= ConstraintSystemFlags::DisableMacroExpansions;
 
+  if (Context.TypeCheckerOpts.EnableConstraintSolverPerformanceHacks)
+    csOptions |= ConstraintSystemFlags::EnablePerformanceHacks;
+
   ConstraintSystem cs(dc, csOptions, diagnosticTransaction);
 
   if (auto *expr = target.getAsExpr()) {