Remove transitive includes in libcpp

[knopp]

This should make it easier to spot missing includes causing problems on internal builds such has flutter/engine#38332.

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[cbracken]

@chinmaygarde Looks like the the overlaid __config_site file was added in #588. Are we manually rolling from an upstream source? If so, we probably want to avoid patching it here in order to avoid it getting stomped on the next roll. Maybe cleaner to add the define in a globally used config in GN instead.

The intent here is to get our config closer to the google3 config. We recently had an internal-only breakage b/262764490 (fix) due to a missing #include <vector> that was transitively included.

Would be nice to eventually add iwyu to the lint, but this at least covers libcxx includes.

[cbracken]

/cc @jason-simmons as well, who's familiar with our process for rolling our libcxx.

[jason-simmons]

__config_site is part of Flutter's buildroot and is not copied from any upstream source. It contains customized flags for configuring Flutter's use of libcxx.