Auto merge of #43612 - michaelwoerister:fix-cgu-hashing, r=eddyb

Auto merge of #43612 - michaelwoerister:fix-cgu-hashing, r=eddyb

incr.comp.: Properly incorporate symbol linkage and visibility into CGU hash.

This PR fixes the way the CGU hash for incr. comp. is computed. The CGU hash represents which `TransItems` are emitted into which codegen unit with which linkage and visibility. Before the new, LLVM-independent symbol internalizer the CGU hash did not accurately contain `TransItem` linkage and visibility because we would not enable symbol internalization in incremental mode anyway. The new internalizer is also run in incremental mode which uncovered the inaccuracy of CGU hashing. Luckily, the fix is rather simple.

r? @eddyb
cc @nikomatsakis