Fix ICE when const closure appears inside a non-const trait method

[lapla-cogito]

Fixes #153891

hir_body_const_context() unconditionally delegated to the parent's const context for const closures, returning None when the parent had no const context. This caused mir_const_qualif() to hit a span_bug!, since mir_promoted() had already decided to call it based on the closure's own syntactic constness. Fall back to ConstContext::ConstFn when the parent's const context is None, so that the const closure body is still properly const-checked rather than triggering an ICE.

Examining another attempt at this issue (which has already been closed), I thought that its approach represents a workaround fix to avoid inconsistencies in the caller of mir_promoted(), whereas I think the correct behavior is for hir_body_const_context() itself to return the proper value.

[rustbot]

r? @dingxiangfei2009

rustbot has assigned @dingxiangfei2009.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler
• compiler expanded to 69 candidates
• Random selection from 10 candidates