Add ldc-profgen tool

[JohanEngelen]

Needed for Sample-based PGO. See #4531

[JohanEngelen]

I don't understand the macOS arm64 failure.
The problem is that LLVM is downloaded without asserts enabled, but somehow CMake reports Building LDC with enabled assertions (LDC_ENABLE_ASSERTIONS): ON. Then when building ldc-profgen, there is a link error due to missing debug-only function symbol (asserts enabled, NDEBUG not defined (-UNDEBUG in compile commandline)). However, if this is the problem, then how did this ever work? There'd be an ABI mismatch and LDC should crash..?

Edit: Ah I see the cross build is using llvm-config of the host (not of the cross package). So then it makes sense that asserts are enabled. And perhaps we are just lucky that there are no ABI mismatches happening.

[kinke]

Ah, good catch wrt. llvm-config. I think I've added an LLVM arm64 build with enabled assertions a bit later, for the native Cirrus CI jobs.

There'd be an ABI mismatch and LDC should crash..?

The cross-compiled compiler isn't tested because it's not runnable on the host anyway. So we wouldn't have seen any CI runtime failures for untagged pipelines. So using enabled assertions doesn't make a lot of sense for these cross-builds; for the 1st native compiler it does though, so that we would see assertions while cross-compiling the compiler itself and its druntime/Phobos. But yeah, the host llvm-config on macOS diverging wrt. assertions mode and that potentially causing problems was a side effect that didn't cross my mind at all so far.

[JohanEngelen]

All green now :)