Add basic char*_t support for libc (partial WG14 N2653) (#90360)

This PR implements a part of WG14 N2653:
 - Define C23 char8_t
 - Define C11 char16_t
 - Define C11 char32_t
 
 Missing goals are:
- The type of UTF-8 character literals is changed from unsigned char to
char8_t. (Since UTF-8 character literals already have type unsigned
char, this is not a semantic change).
- New mbrtoc8() and c8rtomb() functions declared in <uchar.h> enable
conversions between multibyte characters and UTF-8.
    - A new ATOMIC_CHAR8_T_LOCK_FREE macro.
    - A new atomic_char8_t typedef name.

5 files changed, 69 insertions, 0 deletions

diff --git a/libc/include/CMakeLists.txt b/libc/include/CMakeLists.txt
index aeef46a..6dea8e5 100644
--- a/libc/include/CMakeLists.txt
+++ b/libc/include/CMakeLists.txt
@@ -603,6 +603,9 @@ add_gen_header(
   DEPENDS
     .llvm_libc_common_h
     .llvm-libc-types.mbstate_t
+    .llvm-libc-types.char8_t
+    .llvm-libc-types.char16_t
+    .llvm-libc-types.char32_t
 )
 
 add_gen_header(
diff --git a/libc/include/llvm-libc-types/CMakeLists.txt b/libc/include/llvm-libc-types/CMakeLists.txt
index 310374f..16e343d 100644
--- a/libc/include/llvm-libc-types/CMakeLists.txt
+++ b/libc/include/llvm-libc-types/CMakeLists.txt
@@ -90,6 +90,21 @@ add_header(tcflag_t HDR tcflag_t.h)
 add_header(struct_termios HDR struct_termios.h DEPENDS .cc_t .speed_t .tcflag_t)
 add_header(__getoptargv_t HDR __getoptargv_t.h)
 add_header(wchar_t HDR wchar_t.h)
+add_header(char8_t HDR char8_t.h)
+add_header(
+    char16_t 
+  HDR 
+    char16_t.h
+  DEPENDS
+    libc.include.llvm-libc-macros.stdint_macros
+)
+add_header(
+    char32_t 
+  HDR 
+    char32_t.h
+  DEPENDS
+    libc.include.llvm-libc-macros.stdint_macros
+)
 add_header(wint_t HDR wint_t.h)
 add_header(sa_family_t HDR sa_family_t.h)
 add_header(socklen_t HDR socklen_t.h)
diff --git a/libc/include/llvm-libc-types/char16_t.h b/libc/include/llvm-libc-types/char16_t.h
new file mode 100644
index 0000000..1f5847a
--- /dev/null
+++ b/libc/include/llvm-libc-types/char16_t.h
@@ -0,0 +1,17 @@
+//===-- Definition of char16_t type ---------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_TYPES_CHAR16_T_H
+#define LLVM_LIBC_TYPES_CHAR16_T_H
+
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L
+#include "../llvm-libc-macros/stdint-macros.h"
+typedef uint_least16_t char16_t;
+#endif
+
+#endif // LLVM_LIBC_TYPES_CHAR16_T_H
diff --git a/libc/include/llvm-libc-types/char32_t.h b/libc/include/llvm-libc-types/char32_t.h
new file mode 100644
index 0000000..20b72dc
--- /dev/null
+++ b/libc/include/llvm-libc-types/char32_t.h
@@ -0,0 +1,17 @@
+//===-- Definition of char32_t type ---------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_TYPES_CHAR32_T_H
+#define LLVM_LIBC_TYPES_CHAR32_T_H
+
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L
+#include "../llvm-libc-macros/stdint-macros.h"
+typedef uint_least32_t char32_t;
+#endif
+
+#endif // LLVM_LIBC_TYPES_CHAR32_T_H
diff --git a/libc/include/llvm-libc-types/char8_t.h b/libc/include/llvm-libc-types/char8_t.h
new file mode 100644
index 0000000..ddadab1
--- /dev/null
+++ b/libc/include/llvm-libc-types/char8_t.h
@@ -0,0 +1,17 @@
+//===-- Definition of char8_t type ----------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_TYPES_CHAR8_T_H
+#define LLVM_LIBC_TYPES_CHAR8_T_H
+
+#if !defined(__cplusplus) && defined(__STDC_VERSION__) &&                      \
+    __STDC_VERSION__ >= 202311L
+typedef unsigned char char8_t;
+#endif
+
+#endif // LLVM_LIBC_TYPES_CHAR8_T_H