Make conversion between `BridgedIdentifier` and `Identifier` implicit

[ahoppen]

In CASTBridging.cpp, we have quite a few calls to convertIdentifier. We should check if we can eliminate them.

To do this, we have two options:

• Use C++ interop, remove BridgedIdentifier and pass Identifier everywhere we currently pass BridgedIdentifier
• Define a conversion initializer on Identifier that takes a BridgedIdentifier so that we get implicit conversion from BridgedIdentifier to Identifier.