functions in extern blocks are not recognized as unsafe

[Vlad-Shcherbina]

To reproduce

1. Add the following to VSCode's settings.json:

          "[Default Dark+]": {  // or whatever theme you use
               "rules": {
                   "operator.unsafe": "#f00",
                   "function.unsafe": "#f00",
               }
           }

2. Open the following program:

   unsafe fn u() {}
   
   extern "system" {
       fn e();
   }
   
   pub fn f() {
       unsafe {
           u();
           e();
       }
   }

Expected result

Function names in both invocations should be red.

Reference:

A function declared in an extern block is implicitly unsafe.

Actual result

u is red, but e is not:

---

rust-analyzer version: 2020-06-08 (d696776)