[llvm] [Bazel] Migrate to Bzlmod (PR #88927)

[Jordan Rupprecht via llvm-commits]

================
@@ -318,15 +348,17 @@ cc_library(
     deps = [
         ":config",
         ":Demangle",
-        # We unconditionally depend on the custom LLVM zlib wrapper. This will
-        # be an empty library unless zlib is enabled, in which case it will
-        # both provide the necessary dependencies and configuration defines.
-        "@llvm_zlib//:zlib",
-        # We unconditionally depend on the custom LLVM zstd wrapper. This will
-        # be an empty library unless zstd is enabled, in which case it will
-        # both provide the necessary dependencies and configuration defines.
-        "@llvm_zstd//:zstd",
-    ],
----------------
rupprecht wrote:

I kinda like having this pattern, as it makes the build files less cluttered (fewer selects). Could we re-introduce a wrapper around it, similar to the maybe_pfm target in this file? i.e. something like:

```bzl
cc_library(
    name = "maybe_zlib",
    defines = select({
        ":zlib_enabled": ["LLVM_ENABLE_ZLIB=1"],
        "//conditions:default": [],
    }),
    deps = select({
        ":zlib_enabled": ["@zlib-ng"],
        "//conditions:default": [],
    }),
)
```

https://github.com/llvm/llvm-project/pull/88927