Resolves #115394

1. Move definitions of cross-platform `getc` `ungetc` to `reader.h`.
2. Remove function pointer members to define them once per platform in
`.h`
3. Built in overlay mode in macOS m1
4. Remove `reader.cpp` as it's empty now


Also, full build doesn't yet build on macos m1 AFAIK

This is a part of #97655.

Reverts llvm/llvm-project#98075

bots are broken

This is a part of #97655.

This is step 4 of
https://discourse.llvm.org/t/rfc-customizable-namespace-to-allow-testing-the-libc-when-the-system-libc-is-also-llvms-libc/73079

In a previous patch, the printf writer was rewritten to use a single
writer class with a buffer and a callback hook. This patch refactors
scanf's reader to match conceptually.

Scanf is supposed to return EOF when it fails to make any conversions
and there is an input failure. Previously it would return EOF on a
matching failure, which may be an input failure but can also be a
parsing error.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D139891

This patch adds the basic conversion facilities to scanf as well as unit
tests for them. It also adds scanf_main which will be used for the
eventual scanf entrypoints.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D137376

This is the interface that will be used to read from a file or string in
scanf. This patch also adds the string and file implementations of the
reader, although the file reader is not yet complete since ungetc has
not yet been implemented.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D137117