Support LLVM 21.0 (development branch)

[HertzDevil]

Currently, Crystal waits until a new stable major version of LLVM is released before that version is declared in src/llvm/ext/llvm-versions.txt, the list of supported LLVM versions. This has proved to be a hassle when maintaining Crystal on software repositories that only distribute the latest versions of each package; LLVM x.1 is often scheduled half to a whole month before a corresponding Crystal 1.y release, during which either Crystal does not run at all (LLVM could use versioned library names), or someone has to explicitly backport LLVM support on our master branch to a preceding Crystal package.

With this PR, Crystal now supports the latest LLVM development branch explicitly, in the hopes that unattended transitions from LLVM x.0 to x.1 are smooth. This also means Crystal is now responsible for tracking any upstream LLVM changes as soon as possible.

[straight-shoota]

With this change we should also add a test against LLVM's development branch to LLVM CI.

[HertzDevil]

We have not been escaping our strings properly. The Crystal code a = "\\" will produce the LLVM symbol @"'\\'", which itself must be escaped properly since different assemblers handle improperly escaped ones differently. It seems this is fixed in llvm/llvm-project#138817, and now b = "\\\\" would cause a symbol conflict.