Support LLVM 15

[HertzDevil]

It took us a long time, but here is the final PR for LLVM 15 support. Only the last two commits are unique to this PR, the rest are untouched from feature/llvm-opaque-pointers. Resolves #12743. Resolves #12484.

Some numbers copied from here:

Here are the stats of make clean crystal release=1, followed by 10 runs of make clean_cache && bin/crystal run --stats src/compiler/crystal.cr:

	LLVM 14.0.1 (c6e3116)	LLVM 14.0.1 (cfd9d60)	LLVM 15.0.7 (cfd9d60)
Parse	0.0000s (±21.8%)	0.0000s (±17.3%)	0.0000s (±13.9%)
Semantic (top level)	0.4296s (±3.6%)	0.4528s (±9.0%)	0.4658s (±4.7%)
Semantic (new)	0.0018s (±2.6%)	0.0018s (±12.7%)	0.0019s (±17.5%)
Semantic (type declarations)	0.0308s (±3.2%)	0.0310s (±4.6%)	0.0329s (±16.9%)
Semantic (abstract def check)	0.0127s (±1.7%)	0.0131s (±3.0%)	0.0129s (±2.3%)
Semantic (restrictions augmenter)	0.0101s (±4.4%)	0.0101s (±8.9%)	0.0098s (±6.9%)
Semantic (ivars initializers)	4.5505s (±0.8%)	4.5354s (±0.7%)	6.0346s (±2.6%)
Semantic (cvars initializers)	0.0051s (±3.4%)	0.0053s (±9.4%)	0.0050s (±2.0%)
Semantic (main)	6.3904s (±1.9%)	6.3909s (±2.8%)	5.5233s (±3.2%)
Semantic (cleanup)	0.0006s (±4.2%)	0.0006s (±2.4%)	0.0006s (±2.7%)
Semantic (recursive struct check)	0.0012s (±3.6%)	0.0012s (±2.8%)	0.0012s (±8.6%)
Codegen (crystal)	2.6454s (±9.7%)	2.7394s (±3.5%)	2.8363s (±3.8%)
Codegen (bc+obj)	5.9099s (±2.8%)	6.0044s (±2.3%)	5.1995s (±0.8%)
Codegen (linking)	0.7582s (±1.5%)	0.7576s (±1.4%)	0.3392s (±19.1%)
dsymutil	0.3473s (±2.0%)	0.3486s (±1.6%)	0.3940s (±2.9%)
Macro runs	4.6345s (±1.4%)	4.6609s (±1.9%)	3.0254s (±3.3%)

Plus 1 run in release mode (only the stages beyond Crystal's control are shown because the other times are unaffected):

	LLVM 14.0.1 (c6e3116)	LLVM 14.0.1 (cfd9d60)	LLVM 15.0.7 (cfd9d60)
Codegen (bc+obj)	247.8567s	251.4313s	184.3743
Codegen (linking)	0.4536s	0.4714s	0.3295s
dsymutil	0.6281s	0.6342s	0.5236s

[beta-ziliani]

Besides the formatting of codegen that is failing, the rest is 💚
Thanks Quinton for the awesome work 🙇 !