Error/warning messages should consistently use `let...else`

[carols10cents]

I haven't done exhaustive checking on this, but it looks like most error/warning messages use let...else, for example:

• rust/compiler/rustc_parse/messages.ftl

  Line 299 in 8401398

  parse_extra_if_in_let_else = remove the `if` if you meant to write a `let...else` statement

• rust/compiler/rustc_mir_build/messages.ftl

  Line 191 in 8401398

  mir_build_irrefutable_let_patterns_let_else = irrefutable `let...else` {$count ->

• rust/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs

  Line 683 in 8401398

  err.help("...or use `match` instead of `let...else`");

And, most importantly to me, the next print edition of TRPL is about to go out referring to the feature as let...else.

However, there are some error/warning messages that refer to the feature as let else; I think these should be changed to let...else. Some examples:

• rust/compiler/rustc_parse/src/parser/stmt.rs

  Line 870 in 8401398

  format!("{alternatively}you might have meant to use `let else`"),

• rust/compiler/rustc_mir_build/messages.ftl

  Line 337 in 8401398

  mir_build_suggest_let_else = you might want to use `let else` to handle the {$count ->

There are also a number of places that refer to the feature as let-else, but I'm not sure how many of these are really user facing. Example:

rust/src/doc/style-guide/src/statements.md

Lines 103 to 111 in 8401398

	### else blocks (let-else statements)
	A let statement can contain an `else` component, making it a let-else statement.
	In this case, always apply the same formatting rules to the components preceding
	the `else` block (i.e. the `let pattern: Type = initializer_expr` portion)
	as described [for other let statements](#let-statements).
	Format the entire let-else statement on a single line if all the following are
	true:

[carols10cents]

@rustbot label A-diagnostics

[jieyouxu]

EDIT: nvm, this makes sense

[jieyouxu]

I think it makes sense given appropriate context. There are two forms that we should stick with:

1. The name of the construct is "let-else" (this is what the RFC calls it and other docs like Style Guide / RBE).
2. The "shape" of the construct is let...else.

[reddevilmidzy]

I'd like to work on this issue @rustbot claim