[llvm] RuntimeLibcalls: Add bitset for available libcalls (PR #150869)

[Jakub Kuderski via llvm-commits]

https://github.com/kuhar requested changes to this pull request.

Can you land the bitset changes in a separate PR? We prefer to keep ADT changes isolated since they affect the virtually the whole project, so any reverts rebuilt the whole tree.

https://github.com/llvm/llvm-project/pull/150869