fanquake

Backported to 27.x in #29693.

> I think that is currently failing due to a bug in the llvm upstream build system (https://github.com/bitcoin/bitcoin/pull/28880#issuecomment-1846869958) Right, I was referring to this failure, https://github.com/bitcoin/bitcoin/pull/29078#issuecomment-1863598765, where we can't compile...

Pulled back in the commit to fix Qt for Clang < 17.

I'm just going to squash this back into #21778, and get it done together. The changes (combined with lld) are now working.

> Removal happens at some unknown time after deprecation, No, removal is going to happen pretty immediately after the 27.x branch off.

https://cirrus-ci.com/task/4643666984173568: ```bash test 2023-03-16T20:06:03.567000Z TestFramework (ERROR): Unexpected exception caught during testing Traceback (most recent call last): File "/private/var/folders/v7/fs2b0v3s0lz1n57gj9y4xb5m0000gn/T/cirrus-ci-build/ci/scratch/build/bitcoin-arm64-apple-darwin/test/functional/test_framework/test_framework.py", line 132, in main self.run_test() File "/private/var/folders/v7/fs2b0v3s0lz1n57gj9y4xb5m0000gn/T/cirrus-ci-build/ci/scratch/build/bitcoin-arm64-apple-darwin/test/functional/interface_rest.py", line 372, in run_test resp...

> Can you explain what that means? Is the recommended way to manually override CXXFLAGS and LDFLAGS now? Yea. `-flto` is really no different to `-O2` or any other compile...

This is waiting on https://github.com/google/oss-fuzz/pull/11498 downstream.

In case anyone is unsure, the --enable-lto option, as-is, does work, no broken binaries etc (and we use (thin) LTO in oss-fuzz). Changes 1 & 3 here don't affect the...

> See: https://github.com/hebasto/bitcoin/pull/52. I think the example usage there: > cmake -B build -DLTO=ON -DLTO_FLAGS="-flto" is somewhat helping make my point. Seems odd to have a `LTO=ON` option, but then...