Raise min required LLVM version

[kinke]

As we're about to (have to) support LLVM 22, I think it's time to prune the supported LLVM versions again. The min version is 15 as of LDC v1.42. I'd suggest raising it to LLVM 18, so that we support 5 LLVM versions after adding 22.

Any preferences?

[kinke]

LLVM 18 was the first version where they switched to the x.1 minor for the first stable release. And where they renamed the StringRef methods (from startswith to starts_with etc.). So I think it's a good fit: #5094

[JohanEngelen]

Fyi: Weka is at LDC 1.38, which max supports LLVM 18, and I am working on updating things to use LLVM 18. So LLVM 18 is also nice for our upgrade path to newer compilers at Weka.