[CMake] Deduplicate Halide_LLVM_VERSION and LLVM_PACKAGE_VERSION

[LebedevRI]

As far as i can tell, they are always exactly identical,
and i don't believe the claim that LLVM_PACKAGE_VERSION
doesn't exist in upper scopes is valid.

[alexreinking]

Do all versions set it as a cache variable?

Even if they do, is that a guarantee going forward? Generally, modern CMake packages are moving towards not caching such things so that sibling folders can load different versions. Seems like the kind of thing we'd just need to add back later.

Also, is this every use of Halide_LLVM_VERSION? IIRC that variable gets configure_file'd into our package.

[LebedevRI]

Do all versions set it as a cache variable?

Even if they do, is that a guarantee going forward? Generally, modern CMake packages are moving towards not caching such things so that sibling folders can load different versions. Seems like the kind of thing we'd just need to add back later.

I don't follow at all, but you already rely on LLVM_PACKAGE_VERSION being set into cache variable:
https://github.com/halide/Halide/search?q=LLVM_PACKAGE_VERSION

Also, is this every use of Halide_LLVM_VERSION? IIRC that variable gets configure_file'd into our package.

I mean, can you point me at the other uses?
https://github.com/halide/Halide/search?q=Halide_LLVM_VERSION

[alexreinking]

Ah, I was thinking of packaging/CMakeLists.txt which uses LLVM_PACKAGE_VERSION. Thanks for the clean-up!