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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> In the end, it might be easier to change SmallBitVector to use uint64_t.

Or size_t, it's close enough to uintptr_t 

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