Make rustc thread safe when queries can be used #46596

This is just an issue noting down the things required to make rustc thread safe when queries can be used.

• Make GlobalCtxt implement Sync
• Use scoped thread locals instead of globals, WIP: Less globals #46193
• Misc fixes required to use a rayon thread pool, WIP: Parallelize passes using rayon #46564
• Ensure metadata loaded by LlvmMetadataLoader does not get freed on the wrong thread
• CurrentDepGraph.task_stack has to be made a QueryJob field.
• ParseSess.included_mod_stack might be problematic
• Check that optimization fuel is not used with parallel queries
• Ensure that the err_count() API is not used to discover if errors happen in parallel code
• Find a way to order error messages deterministically so that if a query depends on another query, its error messages appear after the other query
• Make mk_attr_id use a scoped thread local or make it part of ParseSess
• See if we can get rid of locks on TransitiveRelation once old borrowck is gone
• Review libproc_macro for issues, Find out which types should be Send, Sync. Deal with Deref impls for Symbol.
• Run CI with cfg(parallel_queries)
• Refactor away CStore::next_crate_num
• Refactor away FileMap.external_src, FileMap.lines, FileMap.multibyte_chars, and FileMap.non_narrow_chars
• When executing queries, instead of keeping around a lock to the query map, create a job object immediately instead