When any field of a Carbon class is access from C++ for the first time,
all fields are exported as `clang::FieldDecl`s (this is necessary
because clang fields have an internal index that is initialized on first
use).
`ClangDeclStore` now provides bidirectional mapping. This allows looking
up a `ClangDeclId` by `InstId`, so when Carbon class fields are exported
they can be looked up that way.
When creating the C++ thunk, make the parameters references if the
corresponding callee parameters are `ref`s.
When creating the Carbon thunk, tag the call arguments as `ref` if the
corresponding callee parameters are `ref`s.