[parallel-queries] Remove the attribute cache from the CrateStore

[michaelwoerister]

The CStore maintains a cache of decoded item attributes which:

• is an instance of shared mutable state that we'd like to avoid and
• is largely unused at this point because item attributes are also cached via the item_attrs query.

The remaining instances where the CrateMetadata::get_item_attrs() is called directly should not be performance critical. To solve this issue:

• Remove the attribute_cache field from CrateMetadata and make things compile again
• Open a PR so that we can due a performance test in order to verify that this indeed does not regress compile times.

cc @rust-lang/wg-compiler-performance

[whitfin]

Hi @michaelwoerister! I took a shot at this here. From my testing everything appears to work that I can see. If you have any advice I'd appreciate it :)