[libc++] default the allocator arguemnt for most constructor

[philnik777]

Allocators are generally very cheap to copy, so avoiding copies by having separate overloads is not that useful. Defaulting them significanlty reduces the overload set the compiler has to consider and simplifies the code, since we can remove some functions in the future.

[github-actions]

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff origin/main HEAD --extensions ,cpp -- libcxx/include/string libcxx/test/std/utilities/optional/optional.specalg/make_optional_explicit.pass.cpp libcxx/test/std/utilities/optional/optional.specalg/make_optional_explicit_initializer_list.pass.cpp --diff_from_common_commit

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View the diff from clang-format here.

diff --git a/libcxx/include/string b/libcxx/include/string
index ea90be950..133878fa2 100644
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -1070,8 +1070,8 @@ public:
   basic_string(nullptr_t) = delete;
 #  endif
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
-  basic_string(const _CharT* __s, size_type __n, const _Allocator& __a = _Allocator())
+  _LIBCPP_HIDE_FROM_ABI
+  _LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string(const _CharT* __s, size_type __n, const _Allocator& __a = _Allocator())
       _LIBCPP_DIAGNOSE_NULLPTR_IF(__n != 0 && __s == nullptr, " if n is not zero")
       : __alloc_(__a) {
     _LIBCPP_ASSERT_NON_NULL(__n == 0 || __s != nullptr, "basic_string(const char*, n, allocator) detected nullptr");
@@ -1168,8 +1168,8 @@ public:
 #  endif
 
 #  ifndef _LIBCPP_CXX03_LANG
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
-  basic_string(initializer_list<_CharT> __il, const _Allocator& __a = _Allocator())
+  _LIBCPP_HIDE_FROM_ABI
+  _LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string(initializer_list<_CharT> __il, const _Allocator& __a = _Allocator())
       : __alloc_(__a) {
     __init(__il.begin(), __il.end());
   }