Use the bitflags! macro to implement FlagV1

[tamird]

Closes #15738.

r? @alexcrichton this is another go at #24697. It turns out that rustc_bitflags doesn't need libcore, so this works by putting rustc_bitflags at the start of the chain in place of libcore. What do you think?

[alexcrichton]

Morally I feel like something has gone quite astray if libcore depends on something, and I don't think there's really much benefit to using the bitflags macro to generate a implementation. We'd also be highly unlikely to stabilize a bitflags-generated interface for the flags, so either way I'd personally hold off on trying to stabilize these flags until later.

[alexcrichton]

For now I'm going to close this due to the need to probably take a more comprehensive look at this API in terms of what and how the flags are exposed, and that I think we should keep the bitflags API private for now as it's somewhat of an awkward way to expose an API surface. Thanks for the PR regardless, however!