I think many of these bugs are major enough that employers would want them fixed (e.g. not respecting ABI on Windows). A lot of effort has been put into LLVM’s optimizer and code generators so presumably this is something they value.
They track regressions in compile time and other metrics like instruction count for a bunch of benchmark programs on https://llvm-compile-time-tracker.com
I have no data or inside insight, but my feeling is that most contributors only work on what their employer wants, and bugs from the public are not relevant.
Still better that’s its open source than not though!