[NFC] Add some additional features to MultiLevelTemplateArgumentList

These are useful when dealing with multi-depth instantiation in deferred
concepts, so this is split off of that patch.

clang/include/clang/Sema/Template.h

@@ -75,6 +75,8 @@ enum class TemplateSubstitutionKind : char {
   class MultiLevelTemplateArgumentList {
     /// The template argument list at a certain template depth
     using ArgList = ArrayRef<TemplateArgument>;
+    using ArgListsIterator = SmallVector<ArgList, 4>::iterator;
+    using ConstArgListsIterator = SmallVector<ArgList, 4>::const_iterator;
 
     /// The template argument lists, stored from the innermost template
     /// argument list (first) to the outermost template argument list (last).
@@ -121,6 +123,12 @@ enum class TemplateSubstitutionKind : char {
       return TemplateArgumentLists.size();
     }
 
+    // Determine the number of substituted args at 'Depth'.
+    unsigned getNumSubsitutedArgs(unsigned Depth) const {
+      assert(NumretainedOuterLevels <= Depth && Depth < getNumLevels());
+      return TemplateArgumentLists[getNumLevels() - Depth - 1].size();
+    }
+
     unsigned getNumRetainedOuterLevels() const {
       return NumRetainedOuterLevels;
     }
@@ -158,6 +166,14 @@ enum class TemplateSubstitutionKind : char {
       return !(*this)(Depth, Index).isNull();
     }
 
+    bool isAnyArgInstantiationDependent() const {
+      for (ArgList List : TemplateArgumentLists)
+        for (const TemplateArgument &TA : List)
+          if (TA.isInstantiationDependent())
+            return true;
+      return false;
+    }
+
     /// Clear out a specific template argument.
     void setArgument(unsigned Depth, unsigned Index,
                      TemplateArgument Arg) {
@@ -183,6 +199,14 @@ enum class TemplateSubstitutionKind : char {
       TemplateArgumentLists.push_back(Args);
     }
 
+    /// Replaces the current 'innermost' level with the provided argument list.
+    /// This is useful for type deduction cases where we need to get the entire
+    /// list from the AST, but then add the deduced innermost list.
+    void replaceInnermostTemplateArguments(ArgList Args) {
+      assert(TemplateArgumentLists.size() > 0 && "Replacing in an empty list?");
+      TemplateArgumentLists[0] = Args;
+    }
+
     /// Add an outermost level that we are not substituting. We have no
     /// arguments at this level, and do not remove it from the depth of inner
     /// template parameters that we instantiate.
@@ -197,6 +221,16 @@ enum class TemplateSubstitutionKind : char {
     const ArgList &getInnermost() const {
       return TemplateArgumentLists.front();
     }
+    /// Retrieve the outermost template argument list.
+    const ArgList &getOutermost() const {
+      return TemplateArgumentLists.back();
+    }
+    ArgListsIterator begin() { return TemplateArgumentLists.begin(); }
+    ConstArgListsIterator begin() const {
+      return TemplateArgumentLists.begin();
+    }
+    ArgListsIterator end() { return TemplateArgumentLists.end(); }
+    ConstArgListsIterator end() const { return TemplateArgumentLists.end(); }
   };
 
   /// The context in which partial ordering of function templates occurs.