proc_macro: preserve file module spans for inner attrs

[Dnreikronos]

fixes #157881

out-of-line mods with custom inner attrs were sending proc macros a pretty-printed module wrapper. the awkward bit: tokens inside the module picked up the span from the parent mod foo;, so later diags pointed at the decl instead of the file where the code actually came from.

this keeps the wrapper synthetic, but puts the loaded file body back into the token stream with its own spans. it also skips the attr currently being expanded, so we don't replay it by accident. added a small ui repro too, plus the older span-debug output changed because those spans are now real.

imo this is the least annoying fix for now. fwiw, it keeps the old fake-wrapper path instead of trying to make full token capture for file mods happen in this PR.

[rustbot]

The parser was modified, potentially altering the grammar of (stable) Rust
which would be a breaking change.

cc @fmease

[rustbot]

r? @mu001999

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[mu001999]

@rustbot reroll

[zjp-CN]

Could someone review the PR? The fix seems sensible, and works for me.

[mati865]

@rustbot reroll