rustc: Don't allocate a cnum to syntax crates

Syntax-only crates are no longer registered with the cstore, so there's no need
to allocate crate numbers to them. This ends up leaving gaps in the crate
numbering scheme which is not expected in the rest of the compiler.

Closes #13560
auto merge of #13565 : alexcrichton/rust/issue-13560, r=brson

Syntax-only crates are no longer registered with the cstore, so there's no need
to allocate crate numbers to them. This ends up leaving gaps in the crate
numbering scheme which is not expected in the rest of the compiler.

Closes #13560