internal: add prime-caches subcommand

[davidbarsky]

I found this subcommand to be useful for profiling cache priming. Here's one result that came from running ~/.cargo/bin/rust-analyzer prime-caches . on buck2:

1. syntax::parse is 20%
2. hir_expand::db::parse_macro_expansion is 32% of the runtime.
   1. hir_expand::db::macro_expand is 19%.
      1. 13% is hir_expand::declarative::DeclarativeMacroExpander::expand.
      2. 6% is hir_expand::builtin::derive_macro::BuiltinDeriveExpander::expand.
3. hir_expand::db::token_tree_to_syntax_node is 13%.
4. 20% is hir_def::item_tree::ItemTree::file_item_tree_with_source_map_query, with the bulk of it being hir_def::item_tree::lower::Ctx::lower_module_items. Since refactor: Lower type-refs before type inference #19462 landed, this is now roughly ~10%.