[string] Clean up StringObject; Clarify its tagged BridgeObjects. (#14425)

[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.

stdlib/public/core/StringGuts.swift

@@ -60,7 +60,7 @@ struct _StringGuts {
 
   init(rawBits: _RawBitPattern) {
     self.init(
-      object: _StringObject(rawBits: rawBits.0),
+      object: _StringObject(noReallyHereAreTheRawBits: rawBits.0),
       otherBits: rawBits.1)
   }
 }