Fixes link failures when referencing a symbol involving a fingerprint
from a different package.
Previously we included the `Namespace`'s `import_id` as part of its
fingerprint, which caused local and imported namespaces to get different
fingerprints. We now store the `import_id` on the `NameScope` instead of
on the `Namespace` inst to avoid this problem.
Also, when we reach a package-level `NameScopeId`, consistently
fingerprint it as a (package name, library name) pair. Previously the
fingerprinting depended on whether it was imported or not, as an
imported `NameScopeId` had a parent scope (the current package). We need
to include the library name here so that private entities with the same
name in different libraries have different fingerprints.
Instead of treating all C++ code as coming from a single synthetic
`CheckIRId`, track the `SemIR::File` associated with each C++ location.
This is necessary since each `SemIR::File` has a distinct `CppFile` and
therefore distinct `SourceLocation`s and `ClangSourceLocId`s.
Assisted-by: Gemini via Antigravity
Richard Smith