QDoc: Drop string comparison in favor of Clang API call

This amends commit `d31d4735e21d5a8338c305b8e801a07572a93db6`.

The implementation in the aforementioned commit employed a string
comparison to ascertain whether the declared return type was
`auto`. This string comparison is performed twice per case; once for
the function declaration that represents the input to the `\fn`
command, and once for the return type set on the related
`FunctionNode`.

This patch replaces said string comparisons by obtaining the necessary
information from Clang's API, reducing the number of string
allocations and avoiding the two string comparisons entirely.

Task-number: QTBUG-126537
Change-Id: I8a10be7f4eb2754f5c50207b254d977fba2f24ea
Reviewed-by: Topi Reiniö <[email protected]>
(cherry picked from commit 9da54f2)
Reviewed-by: Qt Cherry-pick Bot <[email protected]>

Author: paulwicking

src/qdoc/qdoc/src/qdoc/clangcodeparser.cpp

@@ -864,12 +864,10 @@ CXChildVisitResult ClangVisitor::visitFnSignature(CXCursor cursor, CXSourceLocat
 
                     const clang::Decl* declaration = get_cursor_declaration(cursor);
                     assert(declaration);
-                    const auto function_declaration = declaration->getAsFunction();
-                    if (function_declaration) {
-                        const auto declaredReturnType = QString::fromStdString(function_declaration->getDeclaredReturnType().getAsString());
-                        const QLatin1String autoString{"auto"};
-                        if (fn->returnType() != autoString && declaredReturnType == autoString)
-                            fn->setDeclaredReturnType(declaredReturnType);
+                    if (const auto function_declaration = declaration->getAsFunction()) {
+                        auto declaredReturnType = function_declaration->getDeclaredReturnType();
+                        if (llvm::dyn_cast_if_present<clang::AutoType>(declaredReturnType.getTypePtrOrNull()))
+                            fn->setDeclaredReturnType(QString::fromStdString(declaredReturnType.getAsString()));
                     }
                 }
             } else { // Possibly an implicitly generated special member