Merge LLVM and Bazel infra from the toolchain repository. (#207)

The Bazel bits are collected into a directory and given less confusing
names (I hope). Other than names, everything is a direct copy from the
toolchain repository without any edits.

A few files needed to be merged in:
- `.gitignore`
- `.pre-commit-config.yaml`

Subsequent commits will add relevant C++ infrastructure and then the
source code itself.

.gitmodules

@@ -0,0 +1,11 @@
+# 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
+
+[submodule "third_party/llvm-bazel"]
+	path = third_party/llvm-bazel
+	url = https://github.com/google/llvm-bazel.git
+	fetchRecurseSubmodules = false
+[submodule "third_party/llvm-project"]
+	path = third_party/llvm-project
+	url = https://github.com/llvm/llvm-project.git