[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.
