Merge #6606

6606: Parse unsafe extern block r=lnicola a=dtolnay

`unsafe extern` block is parsed successfully by rustc, which means it is usable in attribute macro input.
https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=6f805556f176d082d87255957f16b5f6

```rust
#[cfg(parse)]
unsafe extern "C++" {
    fn demo();
}
```

```diff
  SOURCE_FILE@0..52
-   ERROR@0..33
+   EXTERN_BLOCK@0..52
      ATTR@0..13
        POUND@0..1 "#"
        L_BRACK@1..2 "["
        PATH@2..5
          PATH_SEGMENT@2..5
            NAME_REF@2..5
              IDENT@2..5 "cfg"
        TOKEN_TREE@5..12
          L_PAREN@5..6 "("
          IDENT@6..11 "parse"
          R_PAREN@11..12 ")"
        R_BRACK@12..13 "]"
      WHITESPACE@13..14 "\n"
      UNSAFE_KW@14..20 "unsafe"
      WHITESPACE@20..21 " "
      ABI@21..33
        EXTERN_KW@21..27 "extern"
        WHITESPACE@27..28 " "
        STRING@28..33 "\"C++\""
-   WHITESPACE@33..34 " "
-   ERROR@34..52
-     L_CURLY@34..35 "{"
-     WHITESPACE@35..40 "\n    "
-     FN@40..50
-       FN_KW@40..42 "fn"
-       WHITESPACE@42..43 " "
-       NAME@43..47
-         IDENT@43..47 "demo"
-       PARAM_LIST@47..49
-         L_PAREN@47..48 "("
-         R_PAREN@48..49 ")"
-       SEMICOLON@49..50 ";"
-     WHITESPACE@50..51 "\n"
-     R_CURLY@51..52 "}"
+     WHITESPACE@33..34 " "
+     EXTERN_ITEM_LIST@34..52
+       L_CURLY@34..35 "{"
+       WHITESPACE@35..40 "\n    "
+       FN@40..50
+         FN_KW@40..42 "fn"
+         WHITESPACE@42..43 " "
+         NAME@43..47
+           IDENT@43..47 "demo"
+         PARAM_LIST@47..49
+           L_PAREN@47..48 "("
+           R_PAREN@48..49 ")"
+         SEMICOLON@49..50 ";"
+       WHITESPACE@50..51 "\n"
+       R_CURLY@51..52 "}"
```

This is of interest for https://github.com/dtolnay/cxx.

Co-authored-by: David Tolnay <dtolnay@gmail.com>