Add File & Range information to tint::Source

This is the first step in improving the error messages produced while parsing.

The `line` and `column` information of `Source` has been moved to `Source::Location`.

`Source::Range` has been added that contains a `Location` interval - allowing error messages to highlight the full region of the error.

The `File` information provides an optional file path, and pre-splits the content into lines. These lines can be used to print the full line containing an error.

This CL contains a few temporary changes that help split up this work, and to ease integration with Tint.

Bug: tint:282
Change-Id: I7aa501b0a9631f286e8e93fd7396bdbe38175727
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/31420
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Reviewed-by: David Neto <dneto@google.com>
Commit-Queue: David Neto <dneto@google.com>

src/source.cc

@@ -0,0 +1,38 @@
+// Copyright 2020 The Tint Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <sstream>
+
+#include "source.h"
+
+namespace tint {
+namespace {
+std::vector<std::string> split_lines(const std::string& str) {
+  std::stringstream stream(str);
+  std::string line;
+  std::vector<std::string> lines;
+  while (std::getline(stream, line, '\n')) {
+    lines.emplace_back(std::move(line));
+  }
+  return lines;
+}
+}  // namespace
+
+Source::File::File(const std::string& file_path,
+                   const std::string& file_content)
+    : path(file_path), content(file_content), lines(split_lines(content)) {}
+
+Source::File::~File() = default;
+
+}  // namespace tint