Mk/Uses/llvm.mk is going to need _LLVM_MK_VALID_VERSIONS to also list 18

[Mark Millard]

In my testing for early problems for my context by trying to
force llvm18 use I ran into the:

_LLVM_MK_VALID_VERSIONS= 10 11 12 13 14 15 16 17

that is in Mk/Uses/llvm.mk .

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Mark Millard
marklmi at yahoo.com