[Sema] Avoid unnecessary copy on MultiLevelTemplateArgumentList. NFC (llvm#84459)

We don't modify the MLTAL parameter in `SetupConstraintScope`, and it is
better if we don't copy the 120-byte object each time we call the
function.

Author: zyn0217

clang/include/clang/Sema/Sema.h

@@ -11607,12 +11607,14 @@ class Sema final {
       LocalInstantiationScope &Scope,
       const MultiLevelTemplateArgumentList &TemplateArgs);
 
-  /// used by SetupConstraintCheckingTemplateArgumentsAndScope to recursively(in
+  /// Used by SetupConstraintCheckingTemplateArgumentsAndScope to recursively(in
   /// the case of lambdas) set up the LocalInstantiationScope of the current
   /// function.
-  bool SetupConstraintScope(
-      FunctionDecl *FD, std::optional<ArrayRef<TemplateArgument>> TemplateArgs,
-      MultiLevelTemplateArgumentList MLTAL, LocalInstantiationScope &Scope);
+  bool
+  SetupConstraintScope(FunctionDecl *FD,
+                       std::optional<ArrayRef<TemplateArgument>> TemplateArgs,
+                       const MultiLevelTemplateArgumentList &MLTAL,
+                       LocalInstantiationScope &Scope);
 
   /// Used during constraint checking, sets up the constraint template argument
   /// lists, and calls SetupConstraintScope to set up the

clang/lib/Sema/SemaConcept.cpp

@@ -586,7 +586,8 @@ bool Sema::addInstantiatedCapturesToScope(
 
 bool Sema::SetupConstraintScope(
     FunctionDecl *FD, std::optional<ArrayRef<TemplateArgument>> TemplateArgs,
-    MultiLevelTemplateArgumentList MLTAL, LocalInstantiationScope &Scope) {
+    const MultiLevelTemplateArgumentList &MLTAL,
+    LocalInstantiationScope &Scope) {
   if (FD->isTemplateInstantiation() && FD->getPrimaryTemplate()) {
     FunctionTemplateDecl *PrimaryTemplate = FD->getPrimaryTemplate();
     InstantiatingTemplate Inst(