[Bazel] Add support for lld

This patch adds a Bazel configuration to build lld. That includes a
BUILD.bazel file to export the libunwind headers for use by lld. Since
the lld target itself requires libxml2 (through WindowsManifest) it's
currently disabled on Buildkite and marked manual, but all the libraries
build.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D107414

Author: GMNGeoffrey

utils/bazel/llvm-project-overlay/libunwind/BUILD.bazel

@@ -0,0 +1,20 @@
+# This file is licensed 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
+
+package(
+    default_visibility = ["//visibility:public"],
+    licenses = ["notice"],
+)
+
+# The ld64 linker and lld-macho use the libunwind headers only for the constant
+# definitions, in order to parse and convert DWARF to the compact encoding.
+cc_library(
+    name = "unwind_headers_only",
+    hdrs = [
+        "include/__libunwind_config.h",
+        "include/libunwind.h",
+        "include/mach-o/compact_unwind_encoding.h",
+    ],
+    strip_include_prefix = "include",
+)