[ADT] Rename identity_cxx20 to identity (llvm#164927)

Now that the old llvm::identity has moved into IndexedMap.h under a
different name, this patch renames identity_cxx20 to identity.  Note
that llvm::identity closely models std::identity from C++20.

Author: kazutakahirata

llvm/include/llvm/ADT/STLForwardCompat.h

@@ -125,7 +125,7 @@ struct detector<std::void_t<Op<Args...>>, Op, Args...> {
 template <template <class...> class Op, class... Args>
 using is_detected = typename detail::detector<void, Op, Args...>::value_t;
 
-struct identity_cxx20 // NOLINT(readability-identifier-naming)
+struct identity // NOLINT(readability-identifier-naming)
 {
   using is_transparent = void;
 

llvm/include/llvm/ADT/SparseMultiSet.h

@@ -82,7 +82,7 @@ namespace llvm {
 /// @tparam SparseT     An unsigned integer type. See above.
 ///
 template <typename ValueT, typename KeyT = unsigned,
-          typename KeyFunctorT = identity_cxx20, typename SparseT = uint8_t>
+          typename KeyFunctorT = identity, typename SparseT = uint8_t>
 class SparseMultiSet {
   static_assert(std::is_unsigned_v<SparseT>,
                 "SparseT must be an unsigned integer type");

llvm/include/llvm/ADT/SparseSet.h

@@ -113,7 +113,7 @@ struct SparseSetValFunctor {
 /// @tparam SparseT     An unsigned integer type. See above.
 ///
 template <typename ValueT, typename KeyT = unsigned,
-          typename KeyFunctorT = identity_cxx20, typename SparseT = uint8_t>
+          typename KeyFunctorT = identity, typename SparseT = uint8_t>
 class SparseSet {
   static_assert(std::is_unsigned_v<SparseT>,
                 "SparseT must be an unsigned integer type");

llvm/include/llvm/CodeGen/ScheduleDAGInstrs.h

@@ -89,7 +89,7 @@ namespace llvm {
   /// allocated once for the pass. It can be cleared in constant time and reused
   /// without any frees.
   using RegUnit2SUnitsMap =
-      SparseMultiSet<PhysRegSUOper, unsigned, identity_cxx20, uint16_t>;
+      SparseMultiSet<PhysRegSUOper, unsigned, identity, uint16_t>;
 
   /// Track local uses of virtual registers. These uses are gathered by the DAG
   /// builder and may be consulted by the scheduler to avoid iterating an entire

llvm/include/llvm/ExecutionEngine/Orc/Shared/ExecutorAddress.h

@@ -34,7 +34,7 @@ using ExecutorAddrDiff = uint64_t;
 class ExecutorAddr {
 public:
   /// A wrap/unwrap function that leaves pointers unmodified.
-  using rawPtr = llvm::identity_cxx20;
+  using rawPtr = llvm::identity;
 
 #if __has_feature(ptrauth_calls)
   template <typename T> class PtrauthSignDefault {

llvm/lib/CodeGen/RegAllocFast.cpp

@@ -211,7 +211,7 @@ class RegAllocFastImpl {
     unsigned getSparseSetIndex() const { return VirtReg.virtRegIndex(); }
   };
 
-  using LiveRegMap = SparseSet<LiveReg, unsigned, identity_cxx20, uint16_t>;
+  using LiveRegMap = SparseSet<LiveReg, unsigned, identity, uint16_t>;
   /// This map contains entries for each virtual register that is currently
   /// available in a physical register.
   LiveRegMap LiveVirtRegs;

llvm/unittests/ADT/STLForwardCompatTest.cpp

@@ -185,7 +185,7 @@ TEST(TransformTest, ToUnderlying) {
 }
 
 TEST(STLForwardCompatTest, IdentityCxx20) {
-  llvm::identity_cxx20 identity;
+  llvm::identity identity;
 
   // Test with an lvalue.
   int X = 42;