silence warnings in bazel (#1745)

* Split tests into two packages in Bazel build.

This separates MLIR tests and regular Enzyme integration tests into independent
packages, making sure only a subset of targets is rebuilt and retested when
requested to decrease overhead.

* silence warnings in bazel

There are a bunch of these in the codebase

enzyme/BUILD

@@ -161,6 +161,8 @@ cc_library(
  "-DENZYME_VERSION_MAJOR=0",
  "-DENZYME_VERSION_MINOR=0",
  "-DENZYME_VERSION_PATCH=79",
+ "-Wno-unused-variable",
+ "-Wno-return-type",
  ],
  data = ["@llvm-project//clang:builtin_headers_gen"],
  visibility = ["//visibility:public"],