Revert "[libcxx] Define _LIBCPP_HAS_C8RTOMB_MBRTOC8 to true if compiling with clang" (llvm#165268)

Reverts llvm#152724

The PR was merged with broken pre-commit CI.

Author: philnik777

libcxx/include/__config

@@ -1021,9 +1021,7 @@ typedef __char32_t char32_t;
 // the latter depends on internal GNU libc details that are not appropriate
 // to depend on here, so any declarations present when __cpp_char8_t is not
 // defined are ignored.
-#  if defined(__clang__)
-#    define _LIBCPP_HAS_C8RTOMB_MBRTOC8 1
-#  elif defined(_LIBCPP_GLIBC_PREREQ)
+#  if defined(_LIBCPP_GLIBC_PREREQ)
 #    if _LIBCPP_GLIBC_PREREQ(2, 36) && defined(__cpp_char8_t)
 #      define _LIBCPP_HAS_C8RTOMB_MBRTOC8 1
 #    else

libcxx/test/std/depr/depr.c.headers/uchar_h.compile.pass.cpp

@@ -23,6 +23,11 @@
 // __STDC_UTF_16__ may or may not be defined by the C standard library
 // __STDC_UTF_32__ may or may not be defined by the C standard library
 
+#if !defined(TEST_HAS_NO_C8RTOMB_MBRTOC8)
+ASSERT_SAME_TYPE(size_t, decltype(mbrtoc8((char8_t*)0, (const char*)0, (size_t)0, (mbstate_t*)0)));
+ASSERT_SAME_TYPE(size_t, decltype(c8rtomb((char*)0, (char8_t)0, (mbstate_t*)0)));
+#endif
+
 ASSERT_SAME_TYPE(size_t, decltype(mbrtoc16((char16_t*)0, (const char*)0, (size_t)0, (mbstate_t*)0)));
 ASSERT_SAME_TYPE(size_t, decltype(c16rtomb((char*)0, (char16_t)0, (mbstate_t*)0)));
 

libcxx/test/std/depr/depr.c.headers/uchar_h_char8_t.compile.pass.cpp

@@ -23,6 +23,11 @@
 // __STDC_UTF_16__ may or may not be defined by the C standard library
 // __STDC_UTF_32__ may or may not be defined by the C standard library
 
+#if !defined(TEST_HAS_NO_C8RTOMB_MBRTOC8)
+ASSERT_SAME_TYPE(std::size_t, decltype(std::mbrtoc8((char8_t*)0, (const char*)0, (size_t)0, (mbstate_t*)0)));
+ASSERT_SAME_TYPE(std::size_t, decltype(std::c8rtomb((char*)0, (char8_t)0, (mbstate_t*)0)));
+#endif
+
 ASSERT_SAME_TYPE(std::size_t, decltype(std::mbrtoc16((char16_t*)0, (const char*)0, (size_t)0, (mbstate_t*)0)));
 ASSERT_SAME_TYPE(std::size_t, decltype(std::c16rtomb((char*)0, (char16_t)0, (mbstate_t*)0)));
 

libcxx/test/std/strings/c.strings/cuchar.compile.pass.cpp

@@ -0,0 +1,30 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03
+
+#include <uchar.h>
+
+#include "test_macros.h"
+
+// When C++ char8_t support is not enabled, definitions of these functions that
+// match the C2X declarations may still be present in the global namespace with
+// a char8_t typedef substituted for the C++ char8_t type. If so, these are not
+// the declarations we are looking for, so don't test for them.
+#if !defined(TEST_HAS_NO_CHAR8_T)
+using U = decltype(::c8rtomb);
+using V = decltype(::mbrtoc8);
+#  if !_LIBCPP_HAS_C8RTOMB_MBRTOC8
+// expected-error@-3 {{no member named 'c8rtomb' in the global namespace}}
+// expected-error@-3 {{no member named 'mbrtoc8' in the global namespace}}
+#  else
+// expected-no-diagnostics
+#  endif
+#else
+// expected-no-diagnostics
+#endif

libcxx/test/std/strings/c.strings/cuchar_char8_t.compile.pass.cpp

@@ -293,23 +293,6 @@ def _mingwSupportsModules(cfg):
           """,
         ),
     ),
-    # Check for Glibc < 2.36, where there was no support for char8_t functions
-    Feature(
-        name="glibc-no-char8_t-support",
-        when=lambda cfg: "__GLIBC__" in compilerMacros(cfg)
-        and not sourceBuilds(
-            cfg,
-            """
-            #include <uchar.h>
-            #include <wchar.h>
-            int main(void) {
-                char8_t c;
-                mbstate_t s = {0};
-                return mbrtoc8(&c, "", 0, &s);
-            }
-            """,
-        ),
-    ),
     Feature(
         name="has-unix-headers",
         when=lambda cfg: sourceBuilds(