random Empty/Tombstone LLVM assertion error on i686-msvc CI

[ehuss]

There have been a few instances of the following error on i686-msvc on CI:

Assertion failed: !KeyInfoT::isEqual(Val, EmptyKey) && !KeyInfoT::isEqual(Val, TombstoneKey) && "Empty/Tombstone value shouldn't be inserted into map!", file C:\a\rust\rust\src\llvm-project\llvm\include\llvm/ADT/DenseMap.h, line 624

Examples:

• Add IMPLIED_BOUNDS_ENTAILMENT lint #105575 (comment)
• don't splice from files into pipes in io::copy #108283 (comment)

[klensy]

Similar, but on i686-gnu runner: #123550 (comment)

[jieyouxu]

This also happened on i686-gnu-nopt-1 in #133978 (comment)