Sema: Cache Solution::getTotalMemory()

This is showing up in performance profiles.

Author: slavapestov

include/swift/Sema/ConstraintSystem.h

@@ -1474,6 +1474,9 @@ class Solution {
   /// The fixed score for this solution.
   Score FixedScore;
 
+  /// The total memory used by this solution.
+  std::optional<size_t> TotalMemory;
+
 public:
   /// Create a solution for the given constraint system.
   Solution(ConstraintSystem &cs, const Score &score)

lib/Sema/ConstraintSystem.cpp

@@ -1868,7 +1868,11 @@ static inline size_t size_in_bytes(const T &x) {
 }
 
 size_t Solution::getTotalMemory() const {
-  return sizeof(*this) + typeBindings.getMemorySize() +
+  if (TotalMemory)
+    return *TotalMemory;
+
+  const_cast<Solution *>(this)->TotalMemory
+       = sizeof(*this) + typeBindings.getMemorySize() +
          overloadChoices.getMemorySize() +
          ConstraintRestrictions.getMemorySize() +
          (Fixes.size() * sizeof(void *)) + DisjunctionChoices.getMemorySize() +
@@ -1892,6 +1896,8 @@ size_t Solution::getTotalMemory() const {
          size_in_bytes(argumentLists) +
          size_in_bytes(ImplicitCallAsFunctionRoots) +
          size_in_bytes(SynthesizedConformances);
+
+  return *TotalMemory;
 }
 
 DeclContext *Solution::getDC() const { return constraintSystem->DC; }