[llvm][doc][cmake] Use llvm_config instead of llvm_map_components_to_libnames (llvm#34593)

[FurryAcetylCoA]

As discussed in #34593. llvm_map_components_to_libnames can not handle LLVM builds with LLVM_LINK_LLVM_DYLIB. And it's recommended to use llvm_config instead.

However, I question the necessity of checking USE_SHARED within llvm_config. As I can't envision scenarios where USE_SHARED is not desired by users ( which is equivalent to directly calling explicit_llvm_config). See foutrelis' comment.
Given this, I propose combining llvm_config and explicit_llvm_config into one. But this will makes another PR and I'm not sure how to thoroughly test such a change.

closes #34593 closes #82266

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