internal: Preserve resolve_obligations_as_possible temporary allocation across calls

[Veykril]

This saves ~20ms in the highlihting bench on my machine

  284ms - highlight
      150ms - infer:wait @ per_query_memory_usage
          150ms - infer_query
                0   - PerNs::filter_visibility (436 calls)
                0   - crate_def_map:wait (336 calls)
                2ms - deref_by_trait (909 calls)
                0   - generic_params_query (1 calls)
                0   - inherent_impls_in_block_query (1 calls)
              107ms - resolve_obligations_as_possible (17013 calls)
                0   - trait_solve::wait (1017 calls)
        0   - PerNs::filter_visibility (13 calls)
       17ms - Semantics::analyze_impl (19 calls)
        0   - SourceBinder::to_module_def (30 calls)
        0   - attrs_query (6 calls)
        0   - classify_lifetime (1 calls)
        0   - classify_lifetime_ref (4 calls)
       35ms - classify_name (28 calls)
       54ms - classify_name_ref (452 calls)
        0   - crate_def_map:wait (375 calls)
        7ms - descend_into_macros (776 calls)
        0   - generic_params_query (5 calls)
        0   - impl_data_with_diagnostics_query (1 calls)
       17ms - infer:wait (32 calls)
        0   - resolve_obligations_as_possible (1 calls)
        0   - source_file_to_def (1 calls)
        0   - trait_solve::wait (1 calls)

to

  256ms - highlight
      121ms - infer:wait @ per_query_memory_usage
          121ms - infer_query
                0   - PerNs::filter_visibility (436 calls)
                0   - crate_def_map:wait (336 calls)
                2ms - deref_by_trait (909 calls)
                0   - generic_params_query (1 calls)
                0   - inherent_impls_in_block_query (1 calls)
               81ms - resolve_obligations_as_possible (17013 calls)
                0   - trait_solve::wait (1017 calls)
        0   - PerNs::filter_visibility (13 calls)
       17ms - Semantics::analyze_impl (19 calls)
        0   - SourceBinder::to_module_def (30 calls)
        0   - attrs_query (6 calls)
        0   - classify_lifetime (1 calls)
        0   - classify_lifetime_ref (4 calls)
       35ms - classify_name (28 calls)
       56ms - classify_name_ref (452 calls)
        0   - crate_def_map:wait (375 calls)
        7ms - descend_into_macros (776 calls)
        0   - generic_params_query (5 calls)
        0   - impl_data_with_diagnostics_query (1 calls)
       16ms - infer:wait (32 calls)
        0   - resolve_obligations_as_possible (1 calls)
        0   - source_file_to_def (1 calls)
        0   - trait_solve::wait (1 calls)

[lowr]

Seems reasonable, can you put a comment somewhere, maybe a doc comment on the field?

[Veykril]

@bors r+

[bors]

📌 Commit c1c8e78 has been approved by Veykril

It is now in the queue for this repository.

[bors]

⌛ Testing commit c1c8e78 with merge 39ebc37...

[bors]

☀️ Test successful - checks-actions
Approved by: Veykril
Pushing 39ebc37 to master...