Exclude prebuilts/clang/host/linux-x86/clang-dev/BUILD from the symlink forest.

Background: toolchain/llvm_android/test_compiler.py generates clang-dev, a directory
symlink pointing to out/install/linux-x86/clang-dev, which itself
contains a BUILD file. This BUILD file should be excluded from the
symlink forest because it's not an allowlisted BUILD file.

Problem: However, our current symlink forest logic and finder logic doesn't
handle this situation. It's not in bazel.list:

```
$ less out/.module_paths/bazel.list | grep prebuilts/clang
prebuilts/clang/host/linux-x86/BUILD.bazel
prebuilts/clang/host/linux-x86/cc_toolchain_config.bzl
prebuilts/clang/host/linux-x86/cc_toolchain_constants.bzl
prebuilts/clang/host/linux-x86/cc_toolchain_features.bzl
prebuilts/clang/host/linux-x86/clang-r450784d/BUILD.bazel
prebuilts/clang/host/linux-x86/clang-r450784e/BUILD.bazel
prebuilts/clang/host/linux-x86/clang-r458507/BUILD.bazel
prebuilts/clang/host/linux-x86/clang-r468909/BUILD.bazel
prebuilts/clang/host/linux-x86/clang-r468909b/BUILD.bazel
prebuilts/clang/host/linux-x86/clang-r475365/BUILD.bazel
```

The symlink forest logic uses this bazel.list file and matches it
against the keepExistingBuildFile list to exclude BUILD files from the
symlink forest. Since clang-dev/BUILD.bazel is not in bazel.list, it's
symlinked into the forest.

All of these gymnastics could be avoided if we added custom BUILD file
name support to Bazel, and only symlinked (e.g.) Android.bazel and
ignored all other BUILD files in the tree. It would be very clear which
checked-in BUILD files we want to use in the symlink forest.

I think we should pursue custom BUILD file names instead of adding more
complexities, to say, the finder logic, to support such a use case. It's
also why I decided to add this hardcoded exclusion to keep the
workaround simple.

Bug: 260809113
Test: presubmits
Test: $ DIST_DIR=/tmp/dist prebuilts/python/linux-x86/bin/python3 toolchain/llvm_android/test_compiler.py --build-only --target aosp_raven-userdebug --no-clean-built-target --module dist --module droid --module tidy-soong_subset --with-tidy ./
Change-Id: I415371543585c1c5e8e00e6958105f65ea5978ee