Add a fallback when attribute expansion returns a compile error

[Diggsey]

See #10520

Since attribute expansion was enabled by default it means that completion doesn't work anymore inside functions annotated with an attribute.

It would be useful if rust-analyzer could detect this case (where the attribute implementation fails to parse the code) and falls back to the old behaviour in that case.

[flodiebold]

That would require rather brittle special-casing, wouldn't result in correct completions in all cases, and would relieve pressure from proc macro authors to actually make their macros error resilient (which is the only correct fix). IMO we shouldn't do it.

(Plus it would make it impossible for the macro to intentionally expand to a compiler error.)

[Veykril]

Closing in favor of #11014