[ConstraintSystem] Add an option to disable the constraint solver perf hacks.

This is helpful in experimenting with constraint solver changes that
might help us remove some of these unsound options. It's not ever mean
to be enabled, but if we're able to remove the things guarded by the
option we can eventually remove the option.

include/swift/Basic/LangOptions.h

@@ -122,6 +122,9 @@ namespace swift {
     /// completions.
     bool CodeCompleteCallPatternHeuristics = false;
 
+    /// Disable constraint system performance hacks.
+    bool DisableConstraintSolverPerformanceHacks = false;
+
     ///
     /// Flags for use by tests
     ///