proc_macro_path_invoc feature gate should allow path-like invocation of bang macros

[jcsoo]

$ rustc --version
rustc 1.27.0-nightly (ac3c2288f 2018-04-18)

When #[feature(use_extern_macros)] is enabled, invoking bang macros using an explicit path triggers an error:

error[E0658]: paths of length greater than one in macro invocations are currently unstable
    --> /Users/jcsoo/bobbin-dev/bobbin-dev/mcu/bobbin-stm32/stm32f74x/src/irq.rs:1413:1
     |
1413 | ::bobbin_mcu::irq!(::dma::Dma2Stream7, IrqDma, Irq70);
     | ^^^^^^^^^^^^^^^^^
     |
     = help: add #![feature(proc_macro_path_invoc)] to the crate attributes to enable

Per discussion in #50120 (comment), this type of invocation should be allowed.

[alexcrichton]

I believe this has been fixed

[mark-i-m]

@alexcrichton Does this include things like #[derive($crate::serde_serialize::Serialize)]?

[petrochenkov]

@mark-i-m
Multi-segment paths in attributes should still be gated with proc_macro_path_invoc, that includes derive.