Fill in SymbolInformation container_name from workspace/symbol query

[kjeremy]

It would be nice to fill this in for the UI.

[matklad]

Will need to add additional info to FileSymbol struct:

https://github.com/rust-analyzer/rust-analyzer/blob/0974e6abeb9c3f047e21c3e23769b93c9e7dcaf3/crates/ra_ide_api/src/symbol_index.rs#L208-L213

And we should add more context to the to_symbol function such that it actually knows the parent:

https://github.com/rust-analyzer/rust-analyzer/blob/0974e6abeb9c3f047e21c3e23769b93c9e7dcaf3/crates/ra_ide_api/src/symbol_index.rs#L215-L231