assertion failure in the llvm7 branch when building std lib tests for macos

[andrewrk]

zig: /home/andy/downloads/llvm_release_70/include/llvm/Support/Casting.h:255: typename llvm::cast_retty<X, Y*>::ret_type llvm::cast(Y*) [with X = llvm::GlobalVariable; Y = llvm::Value; typename llvm::cast_retty<X, Y*>::ret_type = llvm::GlobalVariable*]: Assertion `isa<X>(Val) && "cast<Ty>() argument of incompatible type!"' failed.
The following command terminated unexpectedly:
./zig test /home/andy/dev/zig-llvm7/std/index.zig --target-arch x86_64 --target-os macosx --target-environ unknown --test-name-prefix std-macosx-x86_64-Debug-bare  -isystem /nix/store/ggdq1si53c6qb0b67lgx145xkndfy4zy-zlib-1.2.11-dev/include -isystem /nix/store/z0v09fk096lbsnq0pmmgwqvl0dmlafdm-libxml2-2.9.8-dev/include -isystem /nix/store/ggdq1si53c6qb0b67lgx145xkndfy4zy-zlib-1.2.11-dev/include -isystem /nix/store/z0v09fk096lbsnq0pmmgwqvl0dmlafdm-libxml2-2.9.8-dev/include -rpath /nix/store/r44986f5n9a290l30n1y2sy6gwbghkjg-tmpAoeu/lib64 -rpath /nix/store/r44986f5n9a290l30n1y2sy6gwbghkjg-tmpAoeu/lib --library-path /nix/store/vm7czzwvpwlsd9hkva2y3pr343vbc4zr-zlib-1.2.11/lib --library-path /nix/store/q0pfi80ji6pdccihqiy1ig18m2hzww7r-libxml2-2.9.8/lib --library-path /nix/store/vm7czzwvpwlsd9hkva2y3pr343vbc4zr-zlib-1.2.11/lib --library-path /nix/store/q0pfi80ji6pdccihqiy1ig18m2hzww7r-libxml2-2.9.8/lib