[libc++] Remove alignment attributes from _LIBCPP_COMPRESSED_PAIR (llvm#158595)

These attributes aren't required anymore, since the potential
overalignemnt of the objects is handled by the surrounding anonymous
struct.

Author: philnik777

libcxx/include/__memory/compressed_pair.h

@@ -28,8 +28,8 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 // understand how it works).                                                                                          //
 // ================================================================================================================== //
 
-// The first member is aligned to the alignment of the second member to force padding in front of the compressed pair
-// in case there are members before it.
+// On GCC, the first member is aligned to the alignment of the second member to force padding in front of the compressed
+// pair in case there are members before it.
 //
 // For example:
 // (assuming x86-64 linux)
@@ -53,6 +53,9 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 // Furthermore, that alignment must be the same as what was used in the old __compressed_pair layout, so we must
 // handle reference types specially since alignof(T&) == alignof(T).
 // See https://llvm.org/PR118559.
+//
+// On Clang, this is unnecessary, since we use anonymous structs instead, which automatically handle the alignment
+// correctly.
 
 #ifndef _LIBCPP_ABI_NO_COMPRESSED_PAIR_PADDING
 
@@ -104,18 +107,15 @@ class __compressed_pair_padding<_ToPad, true> {};
 #  else
 #    define _LIBCPP_COMPRESSED_PAIR(T1, Initializer1, T2, Initializer2)                                                \
       struct {                                                                                                         \
-        _LIBCPP_NO_UNIQUE_ADDRESS                                                                                      \
-        __attribute__((__aligned__(::std::__compressed_pair_alignment<T2>))) T1 Initializer1;                          \
+        _LIBCPP_NO_UNIQUE_ADDRESS T1 Initializer1;                                                                     \
         _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T1> _LIBCPP_CONCAT3(__padding1_, __LINE__, _);      \
         _LIBCPP_NO_UNIQUE_ADDRESS T2 Initializer2;                                                                     \
         _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T2> _LIBCPP_CONCAT3(__padding2_, __LINE__, _);      \
       }
 
 #    define _LIBCPP_COMPRESSED_TRIPLE(T1, Initializer1, T2, Initializer2, T3, Initializer3)                            \
       struct {                                                                                                         \
-        _LIBCPP_NO_UNIQUE_ADDRESS                                                                                      \
-        __attribute__((__aligned__(::std::__compressed_pair_alignment<T2>),                                            \
-                       __aligned__(::std::__compressed_pair_alignment<T3>))) T1 Initializer1;                          \
+        _LIBCPP_NO_UNIQUE_ADDRESS T1 Initializer1;                                                                     \
         _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T1> _LIBCPP_CONCAT3(__padding1_, __LINE__, _);      \
         _LIBCPP_NO_UNIQUE_ADDRESS T2 Initializer2;                                                                     \
         _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T2> _LIBCPP_CONCAT3(__padding2_, __LINE__, _);      \