Rollup merge of #143286 - Muscraft:track-diagnostics-note, r=WaffleLapkin

Make -Ztrack-diagnostics emit like a note

[#t-compiler/diagnostics > Rendering -Ztrack-diagnostics like a note](https://rust-lang.zulipchat.com/#narrow/channel/147480-t-compiler.2Fdiagnostics/topic/Rendering.20-Ztrack-diagnostics.20like.20a.20note/with/526608647)

As discussed on the Zulip thread above, I want to make `-Ztrack-diagnostics` emit like a `note`. This is because I find its current output jarring, and the fact that it gets rendered completely left-aligned, [even in the middle of a snippet](86e05cd300/tests/ui/track-diagnostics/track6.stderr), seems like something that should be changed. Turning it into a `note` seems like the best choice, as it would align it with the rest of the output, and `note` is already used for somewhat similar things, like seeing why a lint was fired.

---

Note: turning `-Ztrack-diagnostics` into a `note` will also make `annotate-snippets` API a bit cleaner

tests/ui/track-diagnostics-clippy.stderr

@@ -3,8 +3,8 @@ error: casting to the same type is unnecessary (`u32` -> `u32`)
    |
 LL |     let b = a as u32;
    |             ^^^^^^^^ help: try: `a`
--Ztrack-diagnostics: created at src/tools/clippy/clippy_lints/src/casts/unnecessary_cast.rs:LL:CC
    |
+   = note: -Ztrack-diagnostics: created at src/tools/clippy/clippy_lints/src/casts/unnecessary_cast.rs:LL:CC
    = note: `-D clippy::unnecessary-cast` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::unnecessary_cast)]`
 
@@ -15,8 +15,8 @@ LL |         let d = 42;
    |         ----------- unnecessary `let` binding
 LL |         d
    |         ^
--Ztrack-diagnostics: created at src/tools/clippy/clippy_lints/src/returns.rs:LL:CC
    |
+   = note: -Ztrack-diagnostics: created at src/tools/clippy/clippy_lints/src/returns.rs:LL:CC
    = note: `-D clippy::let-and-return` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::let_and_return)]`
 help: return the expression directly

tests/ui/track-diagnostics.rs

@@ -8,5 +8,6 @@ struct A;
 struct B;
 const S: A = B;
 //~^ ERROR: mismatched types
+//~| NOTE: created at
 
 fn main() {}

tests/ui/track-diagnostics.stderr

@@ -3,7 +3,8 @@ error[E0308]: mismatched types
    |
 LL | const S: A = B;
    |              ^ expected `A`, found `B`
--Ztrack-diagnostics: created at compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs:LL:CC
+   |
+   = note: -Ztrack-diagnostics: created at compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs:LL:CC
 
 error: aborting due to 1 previous error