Panic when crate graph is empty

I can reliably make rust-analyzer panic index out of bounds: the len is 0 but the index is 0 when there's no crate graph loaded.

This typically occurs when there's a discover command configured which doesn't support the current directory, but I imagine you'd hit it when you have a standalone Rust file too.

The indexing error is here:

https://github.com/rust-lang/rust-analyzer/blob/a879a4cbadc3bd3489ac05d529a2843c294c75fc/crates/hir/src/lib.rs#L4471-L4474

So e.g. when you type 1. without a crate graph, r-a panics when it tries to access the crate of i32:

https://github.com/rust-lang/rust-analyzer/blob/a879a4cbadc3bd3489ac05d529a2843c294c75fc/crates/ide-completion/src/context/analysis.rs#L1016

https://github.com/rust-lang/rust-analyzer/blob/a879a4cbadc3bd3489ac05d529a2843c294c75fc/crates/hir/src/lib.rs#L2406

This feels like the API is the wrong shape, and we shouldn't assume that all types have an associated type (I see HasCrate is implemented for Type).

Start by reproducing the panic with an empty crate graph while typing 1., then inspect the indexing at crates/hir/src/lib.rs#L4471-L4474 and the callers in crates/ide-completion/src/context/analysis.rs#L1016 and crates/hir/src/lib.rs#L2406. Trace the HasCrate assumption for Type and add a regression test demonstrating the empty-graph case. Done means the scenario no longer panics.