[libc] Add stdin definition.

[michaelrj]

LGTM

libc/config/linux/api.td

def StringAPI : PublicAPI<"string.h"> {
   let Macros = [
     NullMacro,
   ];
 }
 
 def StdIOAPI : PublicAPI<"stdio.h"> {
   let Macros = [
     SimpleMacroDef<"stderr", "stderr">,
+    SimpleMacroDef<"stdin", "stdin">,
     SimpleMacroDef<"stdout", "stdout">,
     SimpleMacroDef<"_IOFBF", "0">,
     SimpleMacroDef<"_IOLBF", "1">,
     SimpleMacroDef<"_IONBF", "2">,
     SimpleMacroDef<"EOF", "-1">,
   ];
   let Types = ["size_t", "FILE", "cookie_io_functions_t"];
 }

libc/config/linux/x86_64/entrypoints.txt

list(APPEND TARGET_LIBC_ENTRYPOINTS
     libc.src.stdio.fwrite
     libc.src.stdio.fwrite_unlocked
     libc.src.stdio.fprintf
     libc.src.stdio.printf
     libc.src.stdio.putc
     libc.src.stdio.putchar
     libc.src.stdio.puts
     libc.src.stdio.stderr
+    libc.src.stdio.stdin
     libc.src.stdio.stdout
 
     # stdlib.h entrypoints
     libc.src.stdlib._Exit
     libc.src.stdlib.abort
     libc.src.stdlib.atexit
     libc.src.stdlib.exit
     libc.src.stdlib.getenv

libc/spec/stdc.td

HeaderSpec Math = HeaderSpec<
 
           FunctionSpec<"atanhf", RetValSpec<FloatType>, [ArgSpec<FloatType>]>,
       ]
   >;
 
   HeaderSpec StdIO = HeaderSpec<
       "stdio.h",
       [
+          Macro<"stdin">,
           Macro<"stderr">,
           Macro<"stdout">,
           Macro<"_IOFBF">,
           Macro<"_IOLBF">,
           Macro<"_IONBF">,
           Macro<"EOF">,
       ], // Macros
       [ // Types
HeaderSpec StdIO = HeaderSpec<
               RetValSpec<IntType>,
               [ArgSpec<FILERestrictedPtr>,
                ArgSpec<ConstCharRestrictedPtr>,
                ArgSpec<VarArgType>]
           >,
       ],
       [
           ObjectSpec<
+              "stdin",
+              "FILE *"
+          >,
+          ObjectSpec<
               "stdout",
               "FILE *"
           >,
           ObjectSpec<
               "stderr",
               "FILE *"
           >,
       ]

libc/src/__support/File/file.h

private:
   size_t write_unlocked_fbf(const uint8_t *data, size_t len);
   size_t write_unlocked_nbf(const uint8_t *data, size_t len);
 };
 
 // The implementaiton of this function is provided by the platfrom_file
 // library.
 File *openfile(const char *path, const char *mode);
 
+extern File *stdin;
 extern File *stdout;
 extern File *stderr;
 
 } // namespace __llvm_libc
 
 #endif // LLVM_LIBC_SRC_SUPPORT_OSUTIL_FILE_H

libc/src/__support/File/linux_file.cpp

#endif
 
   void *buffer = malloc(File::DEFAULT_BUFFER_SIZE);
   auto *file = reinterpret_cast<LinuxFile *>(malloc(sizeof(LinuxFile)));
   LinuxFile::init(file, fd, buffer, File::DEFAULT_BUFFER_SIZE, _IOFBF, true,
                   modeflags);
   return file;
 }
 
+constexpr size_t STDIN_BUFFER_SIZE = 512;
+char stdin_buffer[STDIN_BUFFER_SIZE];
+static LinuxFile StdIn(0, stdin_buffer, STDIN_BUFFER_SIZE, _IOFBF, false,
+                       File::ModeFlags(File::OpenMode::READ));
+File *stdin = &StdIn;
+
 constexpr size_t STDOUT_BUFFER_SIZE = 1024;
 char stdout_buffer[STDOUT_BUFFER_SIZE];
 static LinuxFile StdOut(1, stdout_buffer, STDOUT_BUFFER_SIZE, _IOLBF, false,
                         File::ModeFlags(File::OpenMode::APPEND));
 File *stdout = &StdOut;
 
 constexpr size_t STDERR_BUFFER_SIZE = 0;
 static LinuxFile StdErr(2, nullptr, STDERR_BUFFER_SIZE, _IONBF, false,
                         File::ModeFlags(File::OpenMode::APPEND));
 File *stderr = &StdErr;
 
 } // namespace __llvm_libc

libc/src/stdio/CMakeLists.txt

add_entrypoint_object(
   HDRS
     fopencookie.h
   DEPENDS
     libc.include.stdio
     libc.src.__support.File.file
 )
 
 add_entrypoint_object(
+  stdin
+  SRCS
+    stdin.cpp
+  HDRS
+    stdin.h
+  DEPENDS
+    libc.include.stdio
+    libc.src.__support.File.file
+    libc.src.__support.File.platform_file
+)
+
+add_entrypoint_object(
   stdout
   SRCS
     stdout.cpp
   HDRS
     stdout.h
   DEPENDS
     libc.include.stdio
     libc.src.__support.File.file

libc/src/stdio/stdin.h

+//===------------------------------------------------------------*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#error "Do not include this file. Instead include __support/File/file.h."

libc/src/stdio/stdin.cpp

+//===-- Definition of the global stdin object -----------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/__support/File/file.h"
+
+#include <stdio.h>
+
+extern "C" FILE *stdin = reinterpret_cast<FILE *>(__llvm_libc::stdin);