LLVM trunk version out-of-date

[al42and]

Since Jul 26th 2023, LLVM trunk is version 18.

self.__trunk_version in hpccm/building_blocks/llvm.py should be bumped to allow installing Clang 17 or 18.

[samcmill]

I have updated the building block, but the undocumented _trunk_version option can be used to override the default behavior, e.g., llvm(upstream=True, _trunk_version='18').

[al42and]

Thank you, @samcmill.