Fix visibility for //tcmalloc:llvm

[meteorcloudy]

Due to bazelbuild/bazel@cac82cf, future Bazel version will enforce
visibility check on keys of select(). This PR fixes visibilty of
//tcmalloc:llvm so that projects depending on tcmalloc can still be able
to build with Bazel@HEAD.

Related: bazelbuild/bazel#12925

[meteorcloudy]

/cc @ckennelly

[meteorcloudy]

The test failure doesn't seem to be caused by this PR?

[ckennelly]

Since the full context for this isn't immediately apparent, can you describe both in the PR and the commit the problem this is addressing?

Additionally, what circumstances are necessary to reproduce it (to ensure that it doesn't backslide)

[meteorcloudy]

Since the full context for this isn't immediately apparent, can you describe both in the PR and the commit the problem this is addressing?

Done.

Additionally, what circumstances are necessary to reproduce it (to ensure that it doesn't backslide)

To reproduce, you'll need a Bazel binary built at bazelbuild/bazel@cac82cf, you can download a prebuilt one via here, then bazel build //tcmalloc/internal should be able to produce the issue.

[ckennelly]

This has been resolved by 4c76466.