[llvm] [ADT] Bitset: add shift operators, word accessors, and etc (PR #193400)

[Jakub Kuderski via llvm-commits]

kuhar wrote:

Now that the std::array constructor is public + explicit, the TestBitsetUInt64Array subclass is no longer needed for construction and only verifyValue/verifyStorageSize are helpful. Could we drop the ctor, or keep `verify*` as free function…

https://github.com/llvm/llvm-project/pull/193400