media-libs/mesa: Fix LLVM dep to pull exactly one version

media-libs/mesa: Fix LLVM dep to pull exactly one version

Since LLVM is slotted, you can't expect multiple different
dependencies to apply to the same slot. For this reason, we need to
ensure that only one form of dependency (set of USE flags) is applied to
LLVM.

To cover all the necessary variants:

- when using [opencl], enforce LLVM version and USE flags through
  implicit dependency via sys-devel/clang;

- enforce [llvm_targets_AMDGPU] for all variants needing it,
  and the non-AMDGPU version only if neither of them applies.