[ConstraintSystem] Initial implementation of disjunction optimizer

This brings back CSOptimizer from swiftlang#63585

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -491,6 +491,10 @@ class TypeVariableType::Implementation {
   /// literal (represented by `ArrayExpr` and `DictionaryExpr` in AST).
   bool isCollectionLiteralType() const;
 
+  /// Determine whether this type variable represents a result type of a
+  /// function call.
+  bool isFunctionResult() const;
+
   /// Retrieve the representative of the equivalence class to which this
   /// type variable belongs.
   ///
@@ -5401,6 +5405,9 @@ class ConstraintSystem {
   std::optional<std::pair<Constraint *, llvm::TinyPtrVector<Constraint *>>>
   selectDisjunction();
 
+  /// The old method that is only used when performance hacks are enabled.
+  Constraint *selectDisjunctionWithHacks();
+
   /// Pick a conjunction from the InactiveConstraints list.
   ///
   /// \returns The selected conjunction.

lib/Sema/CMakeLists.txt

@@ -14,6 +14,7 @@ add_swift_host_library(swiftSema STATIC
   CSStep.cpp
   CSTrail.cpp
   CSFix.cpp
+  CSOptimizer.cpp
   CSDiagnostics.cpp
   CodeSynthesis.cpp
   CodeSynthesisDistributedActor.cpp