Rollup merge of #100168 - WaffleLapkin:improve_diagnostics_for_missing_type_in_a_cons...

Improve diagnostics for `const a: = expr;`

Adds a suggestion to write a type when there is a colon, but the type is not present.
I've also shrunk spans a little, so the suggestions are a little nicer.

Resolves #100146

r? `@compiler-errors`

diff --cc compiler/rustc_parse/src/parser/item.rs
Simple merge