update to icu has again broken all the macports-clang compilers

[Joshua Root]

On 2020-8-13 00:52 , Ken Cunningham wrote:
> the macports-clang compilers actually have no need for macports' libxml2 I believe, the libxml2 provided by the system is sufficient, and libxml2 itself is optionally used by one tiny component that is not relevant to a bootstrap compiler...
> 
> I have disabled libxml2 in various llvm-related ports already, so perhaps I should just look at removing our libxml2 from all the llvm* ports and maybe solve this forever.

It doesn't matter for newer clangs, libxml2 will be upgraded first
because a dependency is declared, and everything will be fine because
libxml2 builds with either the system clang or clang-3.7. The problem is
that clang-3.7 is linked with libxml2 despite (deliberately) not
depending on it.

- Josh