CMake: Improve usage of `llvm-config`, esp. for consistent linking

This commit reworks the LLVM/Clang/LLD discovery process for CMake. The
biggest changes are that:
  1. We search for LLVM from most preferred directory to least, skipping
     any `llvm-config` that is the wrong version, or that doesn't
     support the requested link mode ("static" or "shared").
  2. `ZIG_PREFER_CLANG_CPP_DYLIB` has been renamed to `ZIG_SHARED_LLVM`,
     to better align with `ZIG_STATIC_LLVM`.
  3. We only search for LLVM in the same directory alongside LLVM.
  4. LLVM's link mode is forwarded to Clang, so that we can look for the
     appropriate shared/static libraries.
  5. We use `--link-static` when querying `--system-libs` from llvm-config,
     so that this will include libz and other dependencies for
     statically linking LLD

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