Our current locale base API is a mix of non-reserved system names that
we incorrectly (re)define and internal functions and macros starting
with __libcpp. This patch introduces a function-based internal interface
to isolate the rest of the code base from that mess, so that we can work
on refactoring how each platform implements the base API in subsequent
patches. This makes it possible to refactor how each platform implements
the base localization API without impacting the rest of the code base.
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