Add test for unnecessary unsafe on proc macro attr

Signed-off-by: Jonathan Brouwer <jonathantbrouwer@gmail.com>

tests/ui/attributes/proc-macro-unsafe.rs

@@ -0,0 +1,9 @@
+//@ proc-macro: external-macro-use.rs
+
+extern crate external_macro_use;
+
+#[unsafe(external_macro_use::a)]
+//~^ ERROR unnecessary `unsafe` on safe attribute
+fn f() {}
+
+fn main() {}

tests/ui/attributes/proc-macro-unsafe.stderr

@@ -0,0 +1,8 @@
+error: unnecessary `unsafe` on safe attribute
+  --> $DIR/proc-macro-unsafe.rs:5:3
+   |
+LL | #[unsafe(external_macro_use::a)]
+   |   ^^^^^^
+
+error: aborting due to 1 previous error
+