[Sema] Clarify diagnostic for type attributes only valid in inheritance clauses

[qflen]

Fixes #88338.

The shared diagnostic typeattr_not_inheritance_clause fires for @unchecked, @preconcurrency, @unsafe, and @nonisolated when used as a type attribute outside an inheritance clause. The old wording, "%0 only applies in inheritance clauses", reads as a universal claim about the attribute, but all four have valid declaration-attribute uses elsewhere (imports, funcs, etc.).

Reworded to "%0 can only be used as a type attribute in inheritance clauses" so the error scopes itself to the type-attribute form.

Changes

• include/swift/AST/DiagnosticsSema.def: updated diagnostic text.
• test/Concurrency/preconcurrency_conformances.swift: updated 4 expectations, added a regression test for the reporter's repro (let _: @preconcurrency @MainActor () -> Void = {}).
• test/Concurrency/concurrent_value_checking_typechecker_errors.swift: updated 2 expectations.

No logic changes. The @retroactive sibling diagnostic is unchanged since its wording is already scoped correctly.

[qflen]

For reviewer context: #88356 (Apr 8) also addresses #88338 with a similar test-expectation update but a different wording choice: "attribute '%0' cannot be applied to a type".

I landed on "%0 can only be used as a type attribute in inheritance clauses" because @preconcurrency (and the sibling attributes) can be applied to types in inheritance clauses, so the "cannot be applied to a type" framing loses the scope the original reporter was asking for. This PR also adds a regression test for the reporter's exact repro. Happy to defer either way.