Supply libIndexStore.dylib in the latest snapshots

[MaxDesiatov]

Currently libIndexStore.dylib is not built with the host toolchain. An attempt to resolve that is available in #1456, but there still issues with header paths.

This dynamic library is required to make the --enable-test-discovery flag work when running swift test, even with packages that target WebAssembly.

[MaxDesiatov]

There were some weird issues with C++ header paths being broken. As far as I understand, building IndexStore requires building and installing libcxx, and the latter apparently interferes with the WASI headers or something like that. I didn't have enough time yet to investigate it thoroughly.

[MaxDesiatov]

@kateinoigakukun points to these build script lines:

official toolchains have C++ headers for host environment but our toolchain missing wasm-DEVELOPMENT-SNAPSHOT-2020-07-08-a/usr/bin/../include/c++/v1 directory

[MaxDesiatov]

Closing this as resolved.