I need to try and patch around Qts buildsystem to keep #21778 moving along (the issue being that even when you tell Qt to build using Clang on Linux, it still calls out to GCC, breaking our ability to have a macOS release build env that doesn't have a GCC toolchain installed, and thus no ld binary). Before trying to patch Qt any further, update to the latest LTS release, and update the current patch set.
Backport of core#29732.
Depends on D17247.