Improve diagnostic for patterns in function pointer types

[JonathanBrouwer]

In preparation for some work for #158499

Diagnostic before:

error[E0642]: patterns aren't allowed in methods without bodies
  --> $DIR/fn-ptr-pattern.rs:4:14
   |
LL |     pat2: fn(1..3: bool),
   |              ^^^^
   |
help: give this argument a name or use an underscore to ignore it
   |
LL -     pat2: fn(1..3: bool),
LL +     pat2: fn(_: bool),
   |

Diagnostic after:

error[E0642]: patterns aren't allowed in function pointer types
  --> $DIR/fn-ptr-pattern.rs:4:14
   |
LL |     pat2: fn(1..3: bool),
   |              ^^^^
   |
help: give this argument a name or use an underscore to ignore it
   |
LL -     pat2: fn(1..3: bool),
LL +     pat2: fn(_: bool),
   |

[rustbot]

The parser was modified, potentially altering the grammar of (stable) Rust
which would be a breaking change.

cc @fmease

[rustbot]

r? @petrochenkov

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The reviewer was selected based on:

• Owners of files modified in this PR: compiler
• compiler expanded to 75 candidates
• Random selection from 17 candidates

[petrochenkov]

@bors r+

[rust-bors]

📌 Commit c1bcbb4 has been approved by petrochenkov

It is now in the queue for this repository.