zig c++: fix compilation that includes any c++ std library file

LLVM 12 included a patch that changed the way availability annotations
are specified. We now have to define the _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS
flag to make sure that we tell the c++ headers that we don't use
visibility annotations.

Related LLVM patch: D90843
24215df8c5Timon Kruiper committed on 3/1/2021, 7:02:20 PM· committed by Andrew Kelleyparent85eb05e
2 files changedLine totals unavailable