Switch LLVM to a subtree (#1021)

git rm third_party/llvm-project
git subtree add --prefix third_party/llvm-project https://github.com/llvm/llvm-project.git dbf0d8118ccbfd09def0974bf671c39e94d0be93 --squash

With some fixes for the compile.

.gitmodules

@@ -2,10 +2,6 @@
 # Exceptions. See /LICENSE for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
-[submodule "third_party/llvm-project"]
-	path = third_party/llvm-project
-	url = https://github.com/llvm/llvm-project.git
-	branch = main
 [submodule "third_party/examples/brotli/original"]
 	path = third_party/examples/brotli/original
 	url = https://github.com/google/brotli

.pre-commit-config.yaml

@@ -141,5 +141,6 @@ repos:
 exclude: |
   (?x)^(
       third_party/examples/.*/carbon/.*|
+      third_party/llvm-project/.*|
       .*/fuzzer_corpus/.*
   )$