[Platform] Update musl modulemap and improve musl platform support.

[al45tair]

We need to declare separate top-level modules for every header in the musl C library. Note that our set-up here does require some tweaks to musl itself (specifically, the alltypes.h header needs some fairly serious massaging to make it module compatible; in particular, we need to pull out every type declaration into its own separate modularized header in bits/types).

Also update Platform.swift a little for more musl support.

rdar://123503615

[al45tair]

This sits on top of #71839.

[al45tair]

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[al45tair]

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[ian-twilightcoder]

Thank you for doing this one!

[ian-twilightcoder]

We also need to change the clang importer to not pass -fbuiltin-headers-in-system-modules when using musl

[al45tair]

We also need to change the clang importer to not pass -fbuiltin-headers-in-system-modules when using musl

That is in this changeset as part of #71844.

[al45tair]

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[al45tair]

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[al45tair]

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