allow where clauses in unit structs

[m4rch3n1ng]

can't get the ci to work so here's another feature. not sure why this would be helpful in real life, but it can be done in stable rust so sure.

diff for #229:

straight up diffing it

--- .tmp/list.txt	2025-04-02 20:15:56.614898554 +0200
+++ .tmp/list.txt.unit-struct-where-clauses	2025-04-02 20:19:44.824375365 +0200
@@ -108,7 +108,6 @@
 tests/ui/const-generics/generic_const_exprs/issue-74634.rs
 tests/ui/const-generics/generic_const_exprs/issue-80561-incorrect-param-env.rs
 tests/ui/const-generics/generic_const_exprs/issue-84408.rs
-tests/ui/const-generics/generic_const_exprs/issue-90847.rs
 tests/ui/const-generics/generic_const_exprs/issue-97047-ice-1.rs
 tests/ui/const-generics/generic_const_exprs/issue-97047-ice-2.rs
 tests/ui/const-generics/generic_const_exprs/nested-abstract-consts-2.rs
@@ -222,7 +221,6 @@
 tests/ui/explicit-tail-calls/ctfe-collatz-multi-rec.rs
 tests/ui/explicit-tail-calls/return-lifetime-sub.rs
 tests/ui/explicit-tail-calls/two-phase.rs
-tests/ui/feature-gates/feature-gate-trivial_bounds-lint.rs
 tests/ui/for-loop-while/loop-break-value.rs
 tests/ui/generic-associated-types/collections.rs
 tests/ui/generic-const-items/assoc-const-AnonConst-ice-108220.rs
@@ -403,8 +401,6 @@
 tests/ui/traits/winnowing/global-non-global-env-3.rs
 tests/ui/traits/winnowing/global-non-global-env-4.rs
 tests/ui/transmutability/enums/uninhabited_optimization.rs
-tests/ui/trivial-bounds/issue-73021-impossible-inline.rs
-tests/ui/trivial-bounds/trivial-bounds-inconsistent.rs
 tests/ui/trivial-bounds/trivial-bounds-object.rs
 tests/ui/try-trait/yeet-for-option.rs
 tests/ui/try-trait/yeet-for-result.rs