The resilience model forces inlinable code to use resiliently conservative access paths for properties in the same module, but some stdlib methods by design escapes storage of a global variable. The additional `Builtin.addressof` validation introduced in #15608 exposed the latent issue here.
Switch StringObject and StringGuts from opaquely storing tagged cocoa
strings into storing small strings. Plumb small string support
throughout the standard library's routines.
In theory there could be a "fixed-layout" enum that's not exhaustive
but promises not to add any more cases with payloads, but we don't
need that distinction today.
(Note that @objc enums are still "fixed-layout" in the actual sense of
"having a compile-time known layout". There's just no special way to
spell that.)
[string] Clean up StringObject; Clarify its tagged BridgeObjects.
Bifurcate StringObjects raw bit-pattern initializers to expose whether
the caller is passing a value rather than a reference. This way,
StringObject can call the approprite BridgeObject helper and
participate in upcoming peephole optimizations like retains of
known-values. This is all meant to be a NOP.
Additionally, do some cleanup while we're at it.
Include the initial implementation of _StringGuts, a 2-word
replacement for _LegacyStringCore. 64-bit Darwin supported, 32-bit and
Linux support in subsequent commits.
