[clang] Reapply "[Clang][Sema] Refactor collection of multi-level template argument lists (#106585)" (PR #111173)

[Erich Keane via cfe-commits]

================
@@ -4159,7 +4159,7 @@ FunctionTemplateDecl *FunctionDecl::getPrimaryTemplate() const {
   if (FunctionTemplateSpecializationInfo *Info
         = TemplateOrSpecialization
             .dyn_cast<FunctionTemplateSpecializationInfo*>()) {
-    return Info->getTemplate();
+    return Info->getTemplate()->getMostRecentDecl();
----------------
erichkeane wrote:

> @mizvekov I reverted the change where `getPrimaryTemplate` returns the most recent declaration & changed `getTemplateInstantiationPattern` to use the most recent declaration of the primary template instead in [d312bd4](https://github.com/llvm/llvm-project/commit/d312bd464bae21d127f468075cb78178f782ca9a).

That seems reasonable to me.

https://github.com/llvm/llvm-project/pull/111173