Add two more aliased tools (#321)

- `clangd` because it's convenient to be able to configure a language
client to use the matching version of clangd that the project is
compiled with
- `llvm-symbolizer` because it's also convenient

toolchain/aliases.bzl

@@ -23,6 +23,8 @@ aliased_tools = [
     "clang-apply-replacements",
     "clang-format",
     "clang-tidy",
+    "clangd",
     "llvm-cov",
     "llvm-profdata",
+    "llvm-symbolizer",
 ]