Span for `unused_macros` lint is too large, which makes writing macros in IDEs annoying

[PatchMixolydic]

Given the following code (playground):

macro_rules! foo {
    () => {};
}

fn bar() {
    // do stuff
}

The current output is:

warning: unused macro definition
 --> src/lib.rs:1:1
  |
1 | / macro_rules! foo {
2 | |     () => {};
3 | | }
  | |_^
  |
  = note: `#[warn(unused_macros)]` on by default

warning: function is never used: `bar`
 --> src/lib.rs:5:4
  |
5 | fn bar() {
  |    ^^^
  |
  = note: `#[warn(dead_code)]` on by default

Note how the span for the unused_macros lint covers the entire macro definition. This is fine for terminal diagnostic output, but when using an IDE or a Rust language server, this causes the entire macro definition to be underlined with yellow:

In my experience, this tends to make writing macros obnoxious without allowing unused_macros (either globally or locally).

Ideally, the span for this lint should be reduced to the macro's identifier, similar to how the dead_code lint's span only highlights bar's identifier:

warning: unused macro definition
 --> src/lib.rs:1:1
  |
1 | macro_rules! foo {
  |              ^^^
  |
  = note: `#[warn(unused_macros)]` on by default

warning: function is never used: `bar`
 --> src/lib.rs:5:4
  |
5 | fn bar() {
  |    ^^^
  |
  = note: `#[warn(dead_code)]` on by default

@rustbot modify labels: +A-macros +C-enhancement

[hellow554]

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