This upstreams https://github.com/apple/llvm-project/pull/8063.
If module FooCore is re-exported through module Foo (by using
`export_as` in the modulemap), look for attributes of FooCore symbols in
Foo.apinotes file.
Swift bundles `std.apinotes` file that adds Swift-specific attributes to
the C++ stdlib symbols. In recent versions of libc++, module std got
split into multiple top-level modules, each of them is re-exported
through std. This change allows us to keep using a single modulemap file
for all supported C++ stdlibs.
rdar://121680760
