Fix a/an typos
auto merge of #13886 : japaric/rust/fix-an-typos, r=alexcrichton

Found the first one in the rust reference docs. I was going to submit a PR with one fix, but figured I could look for more... This is the result.
rustc: Add some suppot for mixing rlibs and dylibs

Currently, rustc requires that a linkage be a product of 100% rlibs or 100%
dylibs. This is to satisfy the requirement that each object appear at most once
in the final output products. This is a bit limiting, and the upcoming libcore
library cannot exist as a dylib, so these rules must change.

The goal of this commit is to enable *some* use cases for mixing rlibs and
dylibs, primarily libcore's use case. It is not targeted at allowing an
exhaustive number of linkage flavors.

There is a new dependency_format module in rustc which calculates what format
each upstream library should be linked as in each output type of the current
unit of compilation. The module itself contains many gory details about what's
going on here.

cc #10729