Fix libclangDoc.a installation, install clang-doc80

Bug 230791 mistakenly put libclangDoc.a under %%DOCS%%. It actually
belongs to clang extras.

Plus, on 8.0.0, the clang-doc binary does work, so we can install that
too.

PORTREVISION bumped for llvm80 due to new file, not bumped for llvm70 as
this plist change only fixes previously broken OPTIONS configurations.

PR: 230791
Submitted by: Greg V
Differential Revision: https://reviews.freebsd.org/D19068