Update LLVM (#2976)

Updates the reference for the STL's llvm-project submodule. Doing so necessitates changes to:
* `basic_string::resize_and_overwrite` to enable a libc++ test to pass
* `pointer_traits::pointer_to` so "fancy pointers" work with `constexpr basic_string` ([LWG-3454](https://cplusplus.github.io/LWG/issue3454))
* Additions and removals to the `expected_results.txt` and `skipped_tests.txt` files that control the external and internal libc++ test runners
* Remove the workaround for [LLVM-53957](llvm/llvm-project#53957) which has been fixed
* Sync `P0088R3_variant`, `P0220R1_any`, and `P220R1_optional` tests with their upstream libc++ sources

Drive-by:
*  sync `tests/std/tests/P0088R3_variant/env.lst` with `usual_17_matrix.lst`

Dual of MSVC-PR-428240.

Author: fsb4000

llvm-project

@@ -4306,7 +4306,9 @@ public:
                 });
         }
 
-        const auto _Result_size = _STD move(_Op)(_Mypair._Myval2._Myptr(), _New_size);
+        auto _Arg_ptr           = _Mypair._Myval2._Myptr();
+        auto _Arg_size          = _New_size;
+        const auto _Result_size = _STD move(_Op)(_Arg_ptr, _Arg_size);
 #if _CONTAINER_DEBUG_LEVEL > 0
         _STL_VERIFY(_Result_size >= 0, "the returned size can't be smaller than 0");
         _STL_VERIFY(_Result_size <= _New_size, "the returned size can't be greater than the passed size");

stl/inc/xstring

@@ -249,7 +249,8 @@ struct pointer_traits {
 
     using _Reftype = conditional_t<is_void_v<element_type>, char, element_type>&;
 
-    _NODISCARD static pointer pointer_to(_Reftype _Val) noexcept(noexcept(_Ty::pointer_to(_Val))) /* strengthened */ {
+    _NODISCARD static _CONSTEXPR20 pointer pointer_to(_Reftype _Val) noexcept(
+        noexcept(_Ty::pointer_to(_Val))) /* strengthened */ { // Per LWG-3454
         return _Ty::pointer_to(_Val);
     }
 };

stl/inc/xutility

@@ -5,5 +5,7 @@
 // REQUIRES: c++11
 // REQUIRES: c++11 || c++14
 // REQUIRES: c++11 || c++14 || c++17 || c++20
+// REQUIRES: has-unix-headers
 // REQUIRES: stdlib=libc++
 // UNSUPPORTED: c++14, c++17, c++2a
+// UNSUPPORTED: msvc

tests/libcxx/expected_results.txt

@@ -3,8 +3,7 @@
 
 RUNALL_INCLUDE ..\universal_prefix.lst
 RUNALL_CROSSLIST
-# TRANSITION, LLVM-53957: _SILENCE_ALL_CXX23_DEPRECATION_WARNINGS belongs to llvm-project/libcxx/test/support/msvc_stdlib_force_include.h
-PM_CL="/EHsc /MTd /std:c++latest /permissive- /FImsvc_stdlib_force_include.h /wd4643 /D_STL_CALL_ABORT_INSTEAD_OF_INVALID_PARAMETER /D_SILENCE_ALL_CXX23_DEPRECATION_WARNINGS /D_SILENCE_NONFLOATING_COMPLEX_DEPRECATION_WARNING /D_USE_JOIN_VIEW_INPUT_RANGE"
+PM_CL="/EHsc /MTd /std:c++latest /permissive- /FImsvc_stdlib_force_include.h /wd4643 /D_STL_CALL_ABORT_INSTEAD_OF_INVALID_PARAMETER /D_USE_JOIN_VIEW_INPUT_RANGE"
 RUNALL_CROSSLIST
 PM_CL="/analyze:autolog- /Zc:preprocessor"
 PM_COMPILER="clang-cl" PM_CL="-fno-ms-compatibility -fno-delayed-template-parsing"

tests/libcxx/magic_comments.txt

@@ -11,7 +11,7 @@ PM_CL="/w14640 /Zc:threadSafeInit-"
 RUNALL_CROSSLIST
 PM_CL="/EHsc /MD /D_ITERATOR_DEBUG_LEVEL=0 /std:c++latest /permissive- /Zc:noexceptTypes-"
 PM_CL="/EHsc /MD /D_ITERATOR_DEBUG_LEVEL=0 /std:c++17 /DCONSTEXPR_NOTHROW /DTEST_PERMISSIVE"
-PM_CL="/EHsc /MD /D_ITERATOR_DEBUG_LEVEL=0 /std:c++20 /permissive-"
+PM_CL="/EHsc /MD /D_ITERATOR_DEBUG_LEVEL=0 /std:c++20"
 PM_CL="/EHsc /MD /D_ITERATOR_DEBUG_LEVEL=1 /std:c++latest /permissive-"
 PM_CL="/EHsc /MD /D_ITERATOR_DEBUG_LEVEL=0 /std:c++latest /permissive- /Zc:char8_t- /Zc:preprocessor"
 PM_CL="/EHsc /MDd /D_ITERATOR_DEBUG_LEVEL=0 /std:c++latest /permissive- /Zc:wchar_t-"
@@ -32,7 +32,7 @@ PM_CL="/BE /c /EHsc /MD /std:c++latest /permissive-"
 PM_CL="/BE /c /EHsc /MDd /std:c++17 /permissive-"
 PM_CL="/BE /c /EHsc /MT /std:c++20 /permissive-"
 PM_CL="/BE /c /EHsc /MTd /std:c++latest /permissive-"
-PM_COMPILER="clang-cl" PM_CL="-fno-ms-compatibility -fno-delayed-template-parsing /EHsc /MD /std:c++20 /permissive-"
+PM_COMPILER="clang-cl" PM_CL="-fno-ms-compatibility -fno-delayed-template-parsing /EHsc /MD /std:c++latest /permissive-"
 PM_COMPILER="clang-cl" PM_CL="-fno-ms-compatibility -fno-delayed-template-parsing /EHsc /MDd /std:c++17 /DTEST_PERMISSIVE"
-PM_COMPILER="clang-cl" PM_CL="-fno-ms-compatibility -fno-delayed-template-parsing /EHsc /MT /std:c++20"
+PM_COMPILER="clang-cl" PM_CL="-fno-ms-compatibility -fno-delayed-template-parsing /EHsc /MT /std:c++20 /permissive-"
 PM_COMPILER="clang-cl" PM_CL="-fno-ms-compatibility -fno-delayed-template-parsing /EHsc /MTd /std:c++latest /permissive- /fp:strict"