Add more tools by default (#463)

Add more tools by default:
* clang-format: Allows to format generated files, so the golden files
can be merged in a compliant format and compared against.
* clang-tidy: Allows to use the toolchain's tidy command out of the box
without needing to install a matching one.
* clangd: Alows to use the toolchain's indexer without needing to
install the matching one.

Interestingly these are available out of the box in WORKSPACE setups.
This change makes them available also for BAZELMOD setups.

toolchain/internal/common.bzl

@@ -27,6 +27,9 @@ _toolchain_tools = {
     name: name
     for name in [
         "clang-cpp",
+        "clang-format",
+        "clang-tidy",
+        "clangd",
         "ld.lld",
         "llvm-ar",
         "llvm-dwp",