fix: `clangd: -32001: invalid AST` (#4164)

Ignore the `-march=*` in `compile_commands.json` which causes issues in
Neovim on Apple M2. This fix is from this
[comment](https://github.com/clangd/clangd/issues/1582#issuecomment-1500031372)

.clangd

@@ -0,0 +1,7 @@
+# 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
+
+CompileFlags:
+  # Workaround for https://github.com/clangd/clangd/issues/1582
+  Remove: [-march=*]