Add a command line flag to mix metadata into the version hash

[brson]

cc #14470

Cargo wants to be in control of deciding which version of a crate to link to, not rustc. Under the current cargo design, rustc does not need to know about version numbers at all except to frob the symbol hashes a bit. If rustc no longer knows about versioning, then it needs to provide some mechanism for mixing versioning metadata into the version hashes.