[Standard Library] [16-bit] StringObject not compiling on 16 bit platforms

[carlos4242]

Description

The implementation of StringObject has a lot of 32 bit and 64 bit specific code and it's not obvious how to adapt it to 16 bit platforms.

Reproduction

This PR can be used to add support for 16 bit pointers in a basic form. Compiling the standard library for embedded cross compile platforms then shows a failure on 16-bit platforms (currently only AVR).

#75127

Expected behavior

Strings should build on 16-bit platforms with suitable internals.

Environment

Swift version 6.0-dev (LLVM 76ba61e88550d3b, Swift fe367f5)
Target: arm64-apple-macosx14.0

Additional information

No response