Update LLVM and switch to upstreamed Bazel BUILD. (#838)

Also removes the legacy llvm-bazel submodule that is no longer needed.

.bazelignore

@@ -4,3 +4,6 @@
 
 # Carbon creates some non-standard bazel directories, so ignore them.
 bazel-carbon-lang
+third_party/llvm-project/libcxx/utils/google-benchmark
+third_party/llvm-project/utils/bazel/examples
+third_party/llvm-project/utils/bazel/llvm-project-overlay

.gitmodules

@@ -2,10 +2,6 @@
 # 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

WORKSPACE

@@ -93,29 +93,28 @@ configure_clang_toolchain(name = "bazel_cc_toolchain")
 # LLVM libraries
 ###############################################################################
 
-local_repository(
-    name = "llvm_bazel",
-    path = "third_party/llvm-bazel/llvm-bazel",
+new_local_repository(
+    name = "llvm-raw",
+    build_file_content = "# empty",
+    path = "third_party/llvm-project",
 )
 
-load("@llvm_bazel//:configure.bzl", "llvm_configure")
+load("@llvm-raw//utils/bazel:configure.bzl", "llvm_configure")
 
 llvm_configure(
     name = "llvm-project",
-    src_path = "third_party/llvm-project",
-    src_workspace = "@carbon//:WORKSPACE",
     targets = [
         "AArch64",
         "X86",
     ],
 )
 
-load("@llvm_bazel//:terminfo.bzl", "llvm_terminfo_system")
+load("@llvm-raw//utils/bazel:terminfo.bzl", "llvm_terminfo_system")
 
 # We require successful detection and use of a system terminfo library.
 llvm_terminfo_system(name = "llvm_terminfo")
 
-load("@llvm_bazel//:zlib.bzl", "llvm_zlib_system")
+load("@llvm-raw//utils/bazel:zlib.bzl", "llvm_zlib_system")
 
 # We require successful detection and use of a system zlib library.
 llvm_zlib_system(name = "llvm_zlib")

common/check.h

@@ -14,7 +14,7 @@ namespace Carbon {
 // Wraps a stream and exiting for fatal errors.
 class ExitingStream {
  public:
-  LLVM_ATTRIBUTE_NORETURN ~ExitingStream() {
+  [[noreturn]] ~ExitingStream() {
     // Finish with a newline.
     llvm::errs() << "\n";
     if (treat_as_bug) {