Add compiler flags -Os and -Oz to optimize binary size

[ysbaddaden]

Follows LLVM defaults. Tajkes advantage that LLVMRunPasses now handles Os and Oz directly, and manually applies the LLVM defaults for LLVM 13 to 16 (new pas manager). The Oz configuration with the legacy manager (LLVM < 13) don't disable the vectorizations; I'm not sure how to do it or if it's worth implementing.

Maybe we'd like to adjust the inliner thresholds to account for Crystal. For example Rust uses 75 and 25. Clang also keeps the SLP vectorization enabled. I didn't check the impact (yet).

Of course this is related to #14393

[straight-shoota]

issue: The man page also needs an update.

[ysbaddaden]

LLVM only added LLVMPassBuilderOptionsSetInlinerThreshold in LLVM 17 😮‍💨

llvm/llvm-project@c20a9bb

[ysbaddaden]

I backported the function. It relies on a C++ class (for unwrap) that is defined in a CPP source file, not in a distributed header file. Luckily the class looks identical between LLVM 13 and 16. That's still kinda ugly to have to copy-paste it 😞

[ysbaddaden]

Wonderful, now it looks like the backport is only compatible with LLVM 16

llvm/llvm-project@4fa3280

[straight-shoota]

Perhaps we could limit support for these new option to LLVM 17+?

[ysbaddaden]

Or keep the legacy pass manager for LLVM < 17? or LLVM < 16 with the backported function.

[ysbaddaden]

I went with a mix of PassManager (legacy) and PassBuilder. While older releases (LLVM <= 12) always use PassManager, and the newest releases (LLVM >= 17) always use PassBuilder, the intermediary releases (LLVM 13 to 16) preferably use PassBuilder but fallback to use the legacy PassManager for Os and Oz.

Hopefully, that should now compile, and avoid to segment the opt levels to the LLVM version that Crystal happens to have been compiled with (all versions from LLVM 12 to 18 compiled on my local) 🤞

Of course it doesn't compile with LLVM 10 on CI 😮‍💨 but it compiles on my local It was because older crystal releases don't support has_method? and/or has_constant? macro methods.