[llvm] [ADT] Add a helper function to create iterators in DenseMap (NFC) (PR #155133)

[Nikita Popov via llvm-commits]

nikic wrote:

FYI this had some peculiar compile-time impact. With GCC as host compiler, it looks like `-g` builds improve and non-`-g` builds regress: https://llvm-compile-time-tracker.com/compare.php?from=60743358d15f19b89e3ef5daa1b28e7b1bef453e&to=e10b619b12c1b0e14c5868113cee2e4432072037&stat=instructions:u No change for stage2 build.

I guess this perturbed some inlining heuristc. Looks ok overall though.

https://github.com/llvm/llvm-project/pull/155133