[libc] Add a TableGen based header generator.

Summary:
* The Python header generator has been removed.
* Docs giving a highlevel overview of the header gen scheme have been
  added.

Reviewers: phosek, abrachet

Subscribers: mgorny, MaskRay, tschuett, libc-commits

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D70197

libc/utils/HdrGen/IncludeFileCommand.cpp

@@ -0,0 +1,50 @@
+//===----------- Implementation of IncludeFileCommand -----------*- 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "IncludeFileCommand.h"
+
+#include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Support/SourceMgr.h"
+
+#include <cstdlib>
+
+namespace llvm_libc {
+
+const char IncludeFileCommand::Name[] = "include_file";
+
+void IncludeFileCommand::run(llvm::raw_ostream &OS, const ArgVector &Args,
+                             llvm::StringRef StdHeader,
+                             llvm::RecordKeeper &Records,
+                             const Command::ErrorReporter &Reporter) const {
+  if (Args.size() != 1) {
+    Reporter.printFatalError(
+        "%%include_file command takes exactly 1 argument.");
+  }
+
+  llvm::StringRef IncludeFile = Args[0];
+  auto Buffer = llvm::MemoryBuffer::getFileAsStream(IncludeFile);
+  if (!Buffer)
+    Reporter.printFatalError(llvm::StringRef("Unable to open ") + IncludeFile);
+
+  llvm::StringRef Content = Buffer.get()->getBuffer();
+
+  // If the included file has %%begin() command listed, then we want to write
+  // only the content after the begin command.
+  // TODO: The way the content is split below does not allow space within the
+  // the parentheses and, before and after the command. This probably is too
+  // strict and should be relaxed.
+  auto P = Content.split("\n%%begin()\n");
+  if (P.second.empty()) {
+    // There was no %%begin in the content.
+    OS << P.first;
+  } else {
+    OS << P.second;
+  }
+}
+
+} // namespace llvm_libc