Merge the source library from the toolchain repository. (#210)

This library manages buffers of source code, either in-memory or mapped
from the filesystem. At the moment it is a bit simplistic and assumes
`mmap` is available in its implementation details. We should make this
more portable in the future, but this is currently just a direct copy
from the toolchain repository.

source/source_buffer_test.cpp

@@ -0,0 +1,59 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "source/source_buffer.h"
+
+#include "gtest/gtest.h"
+#include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/Twine.h"
+#include "llvm/Support/FileSystem.h"
+#include "llvm/Support/raw_ostream.h"
+
+namespace Carbon {
+namespace {
+
+TEST(SourceBufferTest, StringRep) {
+  SourceBuffer buffer =
+      SourceBuffer::CreateFromText(llvm::Twine("Hello") + " World");
+
+  EXPECT_EQ("/text", buffer.Filename());
+  EXPECT_EQ("Hello World", buffer.Text());
+
+  // Give a custom filename.
+  auto buffer2 = SourceBuffer::CreateFromText("Hello World Again!", "/custom/text");
+  EXPECT_EQ("/custom/text", buffer2.Filename());
+  EXPECT_EQ("Hello World Again!", buffer2.Text());
+}
+
+auto CreateTestFile(llvm::StringRef text) -> std::string {
+  int fd = -1;
+  llvm::SmallString<1024> path;
+  auto error_code = llvm::sys::fs::createTemporaryFile("test_file", ".txt", fd, path);
+  if (error_code) {
+    llvm::report_fatal_error(llvm::Twine("Failed to create temporary file: ") +
+                             error_code.message());
+  }
+
+  llvm::raw_fd_ostream out_stream(fd, /*shouldClose=*/true);
+  out_stream << text;
+  out_stream.close();
+
+  return path.str().str();
+}
+
+TEST(SourceBufferTest, FileRep) {
+  auto test_file_path = CreateTestFile("Hello World");
+
+  auto expected_buffer = SourceBuffer::CreateFromFile(test_file_path);
+  ASSERT_TRUE(static_cast<bool>(expected_buffer))
+      << "Error message: " << expected_buffer.takeError();
+
+  SourceBuffer& buffer = *expected_buffer;
+
+  EXPECT_EQ(test_file_path, buffer.Filename());
+  EXPECT_EQ("Hello World", buffer.Text());
+}
+
+}  // namespace
+}  // namespace Carbon