[IRSymtab] Replace linear time lookup with DenseSet (#66376)

There is an inefficiency in the IRSymtab Builder where it does a lookup
of PreservedSymbols when calling addSymbol. This lookup is linear in
time, so it tends to be quite slow. Replacing it with DenseSet gives a
0.1% speedup:

https://llvm-compile-time-tracker.com/compare.php?from=02d27eac0f3f470a93635fc98ae990bf2a9809ed&to=62b09786fff4d53aa0c75b64aea48de241e4a856&stat=instructions:u

This change is quite similar to https://reviews.llvm.org/D157951.