[clang][NFC] Generalize getSpecificAttr for const attributes (#116606)

This patch allows using `getSpecificAttr` for getting `const`
attributes. Previously, if users of this API would want to get a const
Attribute pointer, they had to pass `getSpecificAttr<const XYZ>()`, to
get it compile. It feels like an arbitrary limitation as the constness
was already encoded in the Attribute container's value type.

Author: steakhal

clang/include/clang/AST/AttrIterator.h

@@ -14,11 +14,13 @@
 #define LLVM_CLANG_AST_ATTRITERATOR_H
 
 #include "clang/Basic/LLVM.h"
+#include "llvm/ADT/ADL.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/Support/Casting.h"
 #include <cassert>
 #include <cstddef>
 #include <iterator>
+#include <type_traits>
 
 namespace clang {
 
@@ -113,13 +115,13 @@ inline bool hasSpecificAttr(const Container& container) {
           specific_attr_end<SpecificAttr>(container);
 }
 template <typename SpecificAttr, typename Container>
-inline SpecificAttr *getSpecificAttr(const Container& container) {
-  specific_attr_iterator<SpecificAttr, Container> i =
-      specific_attr_begin<SpecificAttr>(container);
-  if (i != specific_attr_end<SpecificAttr>(container))
-    return *i;
-  else
-    return nullptr;
+inline auto *getSpecificAttr(const Container &container) {
+  using ValueTy = llvm::detail::ValueOfRange<Container>;
+  using ValuePointeeTy = std::remove_pointer_t<ValueTy>;
+  using IterTy = std::conditional_t<std::is_const_v<ValuePointeeTy>,
+                                    const SpecificAttr, SpecificAttr>;
+  auto It = specific_attr_begin<IterTy>(container);
+  return It != specific_attr_end<IterTy>(container) ? *It : nullptr;
 }
 
 } // namespace clang

clang/lib/CodeGen/CGLoopInfo.cpp

@@ -811,7 +811,7 @@ void LoopInfoStack::push(BasicBlock *Header, clang::ASTContext &Ctx,
   // Identify loop attribute 'code_align' from Attrs.
   // For attribute code_align:
   // n - 'llvm.loop.align i32 n' metadata will be emitted.
-  if (const auto *CodeAlign = getSpecificAttr<const CodeAlignAttr>(Attrs)) {
+  if (const auto *CodeAlign = getSpecificAttr<CodeAlignAttr>(Attrs)) {
     const auto *CE = cast<ConstantExpr>(CodeAlign->getAlignment());
     llvm::APSInt ArgVal = CE->getResultAsAPSInt();
     setCodeAlign(ArgVal.getSExtValue());