This patch makes the __config_site header modular, which solves various
problems with non-modular headers. This requires going back to
generating the modulemap file, since we only know how to make
__config_site modular when we're not using the per-target runtime dir.
The patch also adds a test that we support
-Wnon-modular-include-in-module, which warns about non-modular includes
from modules.
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Co-authored-by: Konstantin Varlamov <varconst@apple.com>
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