[libc++][modules] Support using the module std with -fno-char8_t. (llvm#79155)

Exclude some using-declarations in the module purview when compiling
with `-fno-char8_t`.

(cherry picked from commit dc44836)

Author: poyaoc97

libcxx/modules/std/atomic.inc

@@ -60,7 +60,9 @@ export namespace std {
   using std::atomic_char;
   using std::atomic_char16_t;
   using std::atomic_char32_t;
+#ifndef _LIBCPP_HAS_NO_CHAR8_T
   using std::atomic_char8_t;
+#endif
   using std::atomic_int;
   using std::atomic_llong;
   using std::atomic_long;

libcxx/modules/std/iosfwd.inc

@@ -14,7 +14,9 @@ export namespace std {
 #endif
   using std::u16streampos;
   using std::u32streampos;
+#ifndef _LIBCPP_HAS_NO_CHAR8_T
   using std::u8streampos;
+#endif
 
   using std::basic_osyncstream;
   using std::basic_syncbuf;

libcxx/modules/std/string.inc

@@ -34,7 +34,9 @@ export namespace std {
   using std::string;
   using std::u16string;
   using std::u32string;
+#ifndef _LIBCPP_HAS_NO_CHAR8_T
   using std::u8string;
+#endif
 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
   using std::wstring;
 #endif
@@ -58,7 +60,9 @@ export namespace std {
     using std::pmr::string;
     using std::pmr::u16string;
     using std::pmr::u32string;
+#ifndef _LIBCPP_HAS_NO_CHAR8_T
     using std::pmr::u8string;
+#endif
 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
     using std::pmr::wstring;
 #endif

libcxx/modules/std/string_view.inc

@@ -27,7 +27,9 @@ export namespace std {
   using std::string_view;
   using std::u16string_view;
   using std::u32string_view;
+#ifndef _LIBCPP_HAS_NO_CHAR8_T
   using std::u8string_view;
+#endif
 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
   using std::wstring_view;
 #endif