Auto merge of #7159 - Aaron1011:feature/rustdoc-proc-macro-final, r=alexcrichton

Pass --crate-type to rustdoc

This supports the [corresponding rustc PR](https://github.com/rust-lang/rust/pull/62855). To enable rustdoc to properly
document macros, we pass a new flag '--proc-macro-crate' when
documenting a proc-macro crate. This causes rustdoc to enable the
proc-macro compiler logic that runs when rustc is building a proc-macro
crate.

This flag is essentially a more restricted version of
'--crate-type=proc-macro'. I didn't think it was necessary to pass the
full '--crate-type' flag to rustdoc, when only two options would ever be
used (proc-macro vs anything else).