QDoc: Adapt clang/AST/QualTypeNames.h to upstream changes

For compatibility reasons, QDoc carries a custom implementation of
`llvm-project.git/clang/lib/AST/QualTypeNames.cpp`. When QDoc is built
against Clang libraries from LLVM 19, a segmentation fault occurs when
generating the documentation for the Qt 3D module as part of a qt5.git
super-module documentation build.

The segmentation fault is the result of a `nullptr` being passed to
`clang::TypeName::getFullyQualifiedNestedNameSpecifier` for the
`Scope` parameter.

Upon investigation, it became clear that two changes have been made
upstream to the implementation QDoc carries a customized version of,
one of which adds a `nullptr` check. Due to the small footprint of
both changes, this patch applies both of them to QDoc's
`clang/AST/QualTypeNames.h`:

- The upstream change 16832eb58563f77d917198ad9f86db1c2ee162c9 adds a
  `nullptr` check, see llvm/llvm-project#94084
  for details.
- The upstream change 35bfbb3b21e9874d03b730e8ce4eb98b1dcd2d28
  replaces `dyn_cast_or_null<T>(foo)` with `dyn_cast<T>(foo)` for
  never-null arguments. See
  llvm/llvm-project@35bfbb3
  for details.

The changes apply also when QDoc is built against Clang libraries from
LLVM 17 and 18, with both end-to-end tests passing. Given the nature of
the changes, this means these adaptations do not require being wrapped
in `#if LIBCLANG_VERSION_MAJOR` checks.

Fixes: QTBUG-128926
Change-Id: I5863ca213a35042ed325971b42de2bc1e86c6457
Reviewed-by: Luca Di Sera <luca.disera@qt.io>
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>
(cherry picked from commit a2f478b)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

Author: paulwicking

src/qdoc/qdoc/src/qdoc/clang/AST/QualTypeNames.h

@@ -84,8 +84,9 @@ static inline bool getFullyQualifiedTemplateName(const ASTContext &Ctx,
   assert(ArgTDecl != nullptr);
   QualifiedTemplateName *QTName = TName.getAsQualifiedTemplateName();
 
-  if (QTName && !QTName->hasTemplateKeyword()) {
-    NNS = QTName->getQualifier();
+  if (QTName &&
+      !QTName->hasTemplateKeyword() &&
+      (NNS = QTName->getQualifier())) {
     NestedNameSpecifier *QNNS = getFullyQualifiedNestedNameSpecifier(
         Ctx, NNS, WithGlobalNsPrefix);
     if (QNNS != NNS) {
@@ -288,8 +289,8 @@ static inline NestedNameSpecifier *createNestedNameSpecifierForScopeOf(
   assert(Decl);
 
   const DeclContext *DC = Decl->getDeclContext()->getRedeclContext();
-  const auto *Outer = dyn_cast_or_null<NamedDecl>(DC);
-  const auto *OuterNS = dyn_cast_or_null<NamespaceDecl>(DC);
+  const auto *Outer = dyn_cast<NamedDecl>(DC);
+  const auto *OuterNS = dyn_cast<NamespaceDecl>(DC);
   if (Outer && !(OuterNS && OuterNS->isAnonymousNamespace())) {
     if (OuterNS) {
       return createNestedNameSpecifier(Ctx, OuterNS, WithGlobalNsPrefix);