Since `createStubBasedFirSymbolProviderForClassFiles` works for
builtins (after 866368cf), the provider sometimes has to handle
declarations from builtins (`Any?.toString()` or `arrayOf`, for example)
To provide correct origin for such declarations, we have to compute
the origin on the per-declaration basis instead of always setting
`Library` or `BuiltIn` origin
Providing correct origins should improve the consistency of
deserialized fir declarations
To reduce duplication, `getContainerSource` now also uses
`getDeclarationOriginFor`
Roman Golyshev
ece43fe9c6