resolve: Tweak private_macro_use lint to be compatible with upcoming macro prelude changes

[petrochenkov]

Unblocks #139493.
Zulip thread requesting help - #t-compiler/help > Help requested for effects of #139493.

This PR by itself shouldn't cause any observable changes, its only observable effect is that the prelude changes from #139493 will no longer cause regressions in tests like tests/ui/imports/issue-119369.rs or tests/ui/extern/issue-80074.rs.

This is achieved by moving the "is this thing in stdlib prelude" check from an early point (fn process_macro_use_imports) to a later point (fn record_use_inner), at which the stdlib prelude is already populated and can be inspected.
(The is_builtin_macro check is subsumed by the stdlib prelude check, all built-in macros go through the stdlib prelude anyway.)

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[compiler-errors]

Is there some way we can test this behavior? Also for posterity could you explain why this blocks that PR you linked in the description?

[petrochenkov]

@compiler-errors I've updated the PR description.