[clang][AST] Make 'getPartialSpecializations' const-qualified in DeclTemplate

[PragmaTwice]

Summary

I found that ClassTemplateDecl::getSpecializations is const-qualified, but ClassTemplateDecl::getPartialSpecializationsis not, which has very similar implementation to the former method.
So there is a patch to fix it.

By the way, I do not know who is suitable to review my patch, so if there is better choice as reviewers, please add to "reviewers", thanks.

clang/include/clang/AST/DeclTemplate.h

protected:
 
   /// Retrieve the set of specializations of this class template.
   llvm::FoldingSetVector<ClassTemplateSpecializationDecl> &
   getSpecializations() const;
 
   /// Retrieve the set of partial specializations of this class
   /// template.
   llvm::FoldingSetVector<ClassTemplatePartialSpecializationDecl> &
-  getPartialSpecializations();
+  getPartialSpecializations() const;
 
   ClassTemplateDecl(ASTContext &C, DeclContext *DC, SourceLocation L,
                     DeclarationName Name, TemplateParameterList *Params,
                     NamedDecl *Decl)
       : RedeclarableTemplateDecl(ClassTemplate, C, DC, L, Name, Params, Decl) {}
 
   CommonBase *newCommon(ASTContext &C) const override;
 

clang/lib/AST/DeclTemplate.cpp

 
 llvm::FoldingSetVector<ClassTemplateSpecializationDecl> &
 ClassTemplateDecl::getSpecializations() const {
   LoadLazySpecializations();
   return getCommonPtr()->Specializations;
 }
 
 llvm::FoldingSetVector<ClassTemplatePartialSpecializationDecl> &
-ClassTemplateDecl::getPartialSpecializations() {
+ClassTemplateDecl::getPartialSpecializations() const {
   LoadLazySpecializations();
   return getCommonPtr()->PartialSpecializations;
 }
 
 RedeclarableTemplateDecl::CommonBase *
 ClassTemplateDecl::newCommon(ASTContext &C) const {
   auto *CommonPtr = new (C) Common;
   C.addDestruction(CommonPtr);