Bug#1001546: crystal, build-depends on package that is no longer in testing

[Peter Michael Green]

Package: crystal
Version: 1.2.1+dfsg-6
Severity: serious
Justification: rc policy - "Packages must be buildable within the same 
release"

crystal build-depends on libllvm9 or libllvm8, neither of these packages are
available in testing anymore. Please investigate migration to a newer 
version
of llvm.

(Also it's slightly strange to be build-depending on binary libraries 
rather than
dev packages, and also strange to have the build-dependencies without
corrsponding runtime dependencies)