Some drop order tests are needlessly confusing

[jieyouxu]

Suggestion (not for this PR): #2735 refers to

"let _ = foo();" is differently effectful from just "foo();"

This test (tests/ui/drop/issue-2735-2.rs) exercising let _ = <expr>; vs <expr>; drop order does not need to allow non_camel_case_types, just change defer struct to use the canonical name, this is needlessly confusing:

• In the struct decl, defer-the-struct is in type namespace, defer-the-struct-constructor is in value namespace
• In this function decl, defer (fn) is in the value namespace, defer<'_> is in the type namespace
• In the usage in main, defer is the value namespace struct constructor

Same for

• tests/ui/drop/issue-2735-3.rs
• tests/ui/drop/issue-979.rs

Originally posted by @jieyouxu in #138677 (comment)

[shepmaster]

@rustbot claim.