This turns `Cpp` into a keyword, and makes it map to `NameId::Cpp` and
`PackageNameId::Cpp`.
Per discussion with zygoloid, the keyword versus identifier question is
deliberately kept open by #4846. This PR switches to a keyword because
mapping to a specific `PackageNameId` works best with a special `NameId`
not backed by an `IdentifierId`. We could in theory make it work using
`IdentifierId` or a runtime-tracked `PackageNameId` for `Cpp` (e.g.
stored on `SemIR::File`), but this approach is consistent with `Core`
and so seemed like a good starting point.
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Co-authored-by: Dana Jansens <danakj@orodu.net>
This adds support for importing C++ code directly from source rather
than via a `#include`.
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Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
Jon Ross-PerkinsandDana Jansens
Richard SmithandJon Ross-Perkins