[clang] [clang] Optimize castToDeclContext for 2% improvement in build times (PR #76825)

[Nikita Popov via cfe-commits]

nikic wrote:

https://llvm-compile-time-tracker.com/compare.php?from=98624914367bf9091919de330cf322fb6d5e468f&to=7d398e29b0180aacc646fed5406ad7fbd4b30c02&stat=instructions%3Au Nice results for C++ code!

https://github.com/llvm/llvm-project/pull/76825