Property descriptors for static properties were only recently introduced with
SE-438. Since these symbols are not present in Swift libraries that were
compiled with earlier versions of the compiler, it is not safe for
`#_hasSymbol` to check for the property descriptor symbols, since they can be
absent at either link time or runtime.
Resolves rdar://139749275.
Use the `%target-swift-5.1-abi-triple` substitution to compile the tests for
deployment to the minimum OS versions required for use of opaque types, instead
of disabling availability checking.
An early approach to codegen for `#_hasSymbol` relied on the Darwin platfom SDK, but now that the feature lowers directly to NULL checks in LLVM IR a platform restriction is no longer needed.
However, the tests for `#_hasSymbol` remain unsupported on Windows since that OS does not support weak linking.
For each decl that needs a `#_hasSymbol()` query function, emit the corresponding helper function body during IRGen. Use `IRSymbolVisitor` to collect linkable symbols associated with the decl and return true from the helper function if the address of every associated symbol is non-null.
Resolves rdar://101884587
