llvm-core: add version 20.1.6

[wu-vincent]

Summary

Changes to recipe: llvm-core/20.1.6

Motivation

The recipe llvm-openmp/20.1.6 in Conan Center Index depends on llvm-core/20.1.6 when the option build_libomptarget=True is enabled. However, this specific version of llvm-core is currently missing from the index, which prevents users from successfully building llvm-openmp with that option enabled.

Adding llvm-core/20.1.6 resolves this issue and restores the intended functionality of llvm-openmp’s build_libomptarget feature.

Details

This PR introduces the missing recipe for llvm-core/20.1.6.
The version aligns with the existing LLVM 20.1.6 components to ensure compatibility and consistent dependency resolution within the Conan ecosystem.
No functional changes beyond adding the new version; the recipe follows the same structure and conventions as previous llvm-core versions.

---

• Read the [contributing guidelines](https://github.com/conan-io/conan-center-index/blob/master/CONTRIBUTING.md)
• Checked that this PR is not a duplicate: [list of PRs by recipe](List of open pull requests broken down by recipe #24240)
• Tested locally with at least one configuration using a recent version of Conan