Rollup merge of #150491 - petrochenkov:ambigeffvis, r=yaahc

resolve: Mark items under exported ambiguous imports as exported

After rust-lang/rust#147984 one of the imports in an ambiguous import set becomes accessible under a deny-by-default deprecation lint.

So if it points to something, that something needs to be marked as exported, so its MIR is encoded into metadata, its symbol is not lost from object files, etc.
The added test shows an example.
This fixes around 10-20 crater regressions found in rust-lang/rust#149195 (comment).

Unblocks rust-lang/rust#149195.

Author: JonathanBrouwer