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.
If a function's parameter or return types involve nominal types that
have been moved across modules using @_originallyDefinedIn, we must
take care to always mangle the opaque result type's name using the
original module names and not the current module names.
This was a problem with DWARF mangling, which normally disables
@_originallyDefinedIn for other purposes. Make sure to always
temporarily re-enable it when mangling an opaque result type.
Fixes rdar://problem/93822207.
mangleOpaqueTypeDecl() used to enable DWARFMangling, which
ignores @_originallyDefinedIn, which would in turn break module
interfaces.
Fixes rdar://problem/86480663.
