The `-force-single-frontend-invocation` flag predates WMO and is now an
alias for `-whole-module-optimization`. We should use the latter and let
the former fade into history.
Interpreter/multi_payload_extra_inhabitant.swift
Interpreter/struct_extra_inhabitants.swift
Mark the test as unsupported when running the tests with the OS' stdlib.
rdar://54749176
The standard library never ended up needing the low extra inhabitants (<4G on 64-bit Darwin,
<4K elsewhere), so BridgeObject can have the same set of extra inhabitants as the other refcounted
types, allowing `String?????` and `Array??????????` to still use optimized representations.
rdar://problem/45881464
This allows us to layout-optimize Optional<T> when T is a struct with an
extra-inhabitant-bearing field anywhere in its definition, not only at
the beginning. rdar://problem/43019427
