[lldb][test][NFC] Re-arrange ifdefs in compressed_pair.h

In an upcoming patch we'll start supporting a new compressed_pair
layout. This refactor will make it easier to add tests for that new
layout.

(cherry picked from commit 7b22660)
(cherry picked from commit e4daa06)

Author: Michael137

lldb/packages/Python/lldbsuite/test/make/libcxx-simulators-common/compressed_pair.h

@@ -4,6 +4,12 @@
 #include <type_traits>
 #include <utility> // for std::forward
 
+// COMPRESSED_PAIR_REV versions:
+// 0 -> Post-c88580c layout
+// 1 -> Post-27c83382d83dc layout
+// 2 -> Post-769c42f4a552a layout
+// 3 -> padding-less no_unique_address-based layout (introduced in 27c83382d83dc)
+
 namespace std {
 namespace __lldb {
 
@@ -13,7 +19,50 @@ namespace __lldb {
 #define _LLDB_NO_UNIQUE_ADDRESS [[__no_unique_address__]]
 #endif
 
-#if COMPRESSED_PAIR_REV == 0 // Post-c88580c layout
+// From libc++ datasizeof.h
+template <class _Tp> struct _FirstPaddingByte {
+  _LLDB_NO_UNIQUE_ADDRESS _Tp __v_;
+  char __first_padding_byte_;
+};
+
+template <class _Tp>
+inline const size_t __datasizeof_v =
+    __builtin_offsetof(_FirstPaddingByte<_Tp>, __first_padding_byte_);
+
+template <class _Tp>
+struct __lldb_is_final : public integral_constant<bool, __is_final(_Tp)> {};
+
+// The legacy layout has been patched, see
+// https://github.com/llvm/llvm-project/pull/142516.
+#if COMPRESSED_PAIR_REV == 1
+template <class _ToPad> class __compressed_pair_padding {
+  char __padding_[((is_empty<_ToPad>::value &&
+                    !__lldb_is_final<_ToPad>::value) ||
+                   is_reference<_ToPad>::value)
+                      ? 0
+                      : sizeof(_ToPad) - __datasizeof_v<_ToPad>];
+};
+#elif COMPRESSED_PAIR_REV > 1 && COMPRESSED_PAIR_REV < 3
+template <class _ToPad>
+inline const bool __is_reference_or_unpadded_object =
+    (std::is_empty<_ToPad>::value && !__lldb_is_final<_ToPad>::value) ||
+    sizeof(_ToPad) == __datasizeof_v<_ToPad>;
+
+template <class _Tp>
+inline const bool __is_reference_or_unpadded_object<_Tp &> = true;
+
+template <class _Tp>
+inline const bool __is_reference_or_unpadded_object<_Tp &&> = true;
+
+template <class _ToPad, bool _Empty = __is_reference_or_unpadded_object<_ToPad>>
+class __compressed_pair_padding {
+  char __padding_[sizeof(_ToPad) - __datasizeof_v<_ToPad>] = {};
+};
+
+template <class _ToPad> class __compressed_pair_padding<_ToPad, true> {};
+#endif // COMPRESSED_PAIR_REV == 1
+
+#if COMPRESSED_PAIR_REV == 0
 struct __value_init_tag {};
 struct __default_init_tag {};
 
@@ -59,49 +108,6 @@ class __compressed_pair : private __compressed_pair_elem<_T1, 0>,
   _T1 &first() { return static_cast<_Base1 &>(*this).__get(); }
 };
 #elif COMPRESSED_PAIR_REV == 1 || COMPRESSED_PAIR_REV == 2
-// From libc++ datasizeof.h
-template <class _Tp> struct _FirstPaddingByte {
-  _LLDB_NO_UNIQUE_ADDRESS _Tp __v_;
-  char __first_padding_byte_;
-};
-
-template <class _Tp>
-inline const size_t __datasizeof_v =
-    __builtin_offsetof(_FirstPaddingByte<_Tp>, __first_padding_byte_);
-
-template <class _Tp>
-struct __lldb_is_final : public integral_constant<bool, __is_final(_Tp)> {};
-
-// The legacy layout has been patched, see
-// https://github.com/llvm/llvm-project/pull/142516.
-#if COMPRESSED_PAIR_REV == 1
-template <class _ToPad> class __compressed_pair_padding {
-  char __padding_[((is_empty<_ToPad>::value &&
-                    !__lldb_is_final<_ToPad>::value) ||
-                   is_reference<_ToPad>::value)
-                      ? 0
-                      : sizeof(_ToPad) - __datasizeof_v<_ToPad>];
-};
-#else
-template <class _ToPad>
-inline const bool __is_reference_or_unpadded_object =
-    (std::is_empty<_ToPad>::value && !__lldb_is_final<_ToPad>::value) ||
-    sizeof(_ToPad) == __datasizeof_v<_ToPad>;
-
-template <class _Tp>
-inline const bool __is_reference_or_unpadded_object<_Tp &> = true;
-
-template <class _Tp>
-inline const bool __is_reference_or_unpadded_object<_Tp &&> = true;
-
-template <class _ToPad, bool _Empty = __is_reference_or_unpadded_object<_ToPad>>
-class __compressed_pair_padding {
-  char __padding_[sizeof(_ToPad) - __datasizeof_v<_ToPad>] = {};
-};
-
-template <class _ToPad> class __compressed_pair_padding<_ToPad, true> {};
-#endif
-
 #define _LLDB_COMPRESSED_PAIR(T1, Initializer1, T2, Initializer2)              \
   [[__gnu__::__aligned__(                                                      \
       alignof(T2))]] _LLDB_NO_UNIQUE_ADDRESS T1 Initializer1;                  \
@@ -127,7 +133,7 @@ template <class _ToPad> class __compressed_pair_padding<_ToPad, true> {};
   _LLDB_NO_UNIQUE_ADDRESS T1 Name1;                                            \
   _LLDB_NO_UNIQUE_ADDRESS T2 Name2;                                            \
   _LLDB_NO_UNIQUE_ADDRESS T3 Name3
-#endif
+#endif // COMPRESSED_PAIR_REV == 3
 } // namespace __lldb
 } // namespace std