Rollup merge of #61438 - estebank:generics-span, r=varkor

Point at individual type args on arg count mismatch

- Point at individual type arguments on arg count mismatch
- Make generics always have a valid span, even when there are no args
- Explain that `impl Trait` introduces an implicit type argument

Fix #55991.

diff --cc src/libsyntax/parse/parser.rs
Simple merge