GRAM: pin the fn keyword in FnPointer

[afetisov]

Pin fn keyword in function pointer types to improve error recovery.

[vlad20012]

I'm trying to remember why I didn't pin fn pointer when I pin other types (#5050). I think, the reason is this case:

fn foo() -> // caret; a user is typing here

fn bar() {}

An incomplete foo declaration shouldn't break fn bar, I guess.
I think this case can be covered by this grammar:

FnPointerType ::= [ unsafe? ExternAbi? ] fn !identifier FnTypeParameters RetType? {
  pin = 3

[afetisov]

Nice catch, I'll add it to the tests.

[vlad20012]

bors r+

[vlad20012]

bors r-

[bors]

Canceled.

[vlad20012]

bors r+

[bors]

Build succeeded:

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