Rollup merge of rust-lang#147337 - ada4a:write-diag-item, r=fmease

Make `fmt::Write` a diagnostic item

I'm working on an enhancement to [`clippy::format_push_string`](https://rust-lang.github.io/rust-clippy/master/index.html#format_push_string), which would suggest an autofix to replace `push_str`/`+=` with `write!`. But that could require importing `std::fmt::Write`, and so I need this diagnostic item to check if it's already in scope.

The reason I called it `FmtWrite` and not just `Write` is that there's already an `IoWrite` defined.

Author: Zalathar

compiler/rustc_span/src/symbol.rs

@@ -236,6 +236,7 @@ symbols! {
         File,
         FileType,
         FmtArgumentsNew,
+        FmtWrite,
         Fn,
         FnMut,
         FnOnce,

library/core/src/fmt/mod.rs

@@ -115,6 +115,7 @@ pub struct Error;
 /// [`std::io::Write`]: ../../std/io/trait.Write.html
 /// [flushable]: ../../std/io/trait.Write.html#tymethod.flush
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_diagnostic_item = "FmtWrite"]
 pub trait Write {
     /// Writes a string slice into this writer, returning whether the write
     /// succeeded.