Out-of-date clang/llvm version lists in PGAC_LLVM_SUPPORT

I happened to notice that these lists of supported versions haven't
been updated in a good long time:

PGAC_PATH_PROGS(LLVM_CONFIG, llvm-config llvm-config-7 llvm-config-6.0 llvm-config-5.0 llvm-config-4.0 llvm-config-3.9)

PGAC_PATH_PROGS(CLANG, clang clang-7 clang-6.0 clang-5.0 clang-4.0 clang-3.9)

Given the lack of complaints, it seems likely that nobody is relying
on these. Maybe we should just nuke them? If not, I suppose we
better add 8 through 15.

I may be missing it, but it doesn't look like meson.build has any
equivalent lists. So that might be an argument for getting rid
of the lists here?

regards, tom lane