Rollup merge of #144298 - Zalathar:empty-span, r=wesleywiser

coverage: Enlarge empty spans during MIR instrumentation, not codegen

This re-lands the part of rust-lang/rust#140847 that was (hopefully) not responsible for the coverage-instrumentation regressions that caused that PR to be reverted.

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Enlarging empty spans was historically performed during MIR instrumentation, but had to be moved to codegen as part of larger changes in rust-lang/rust#134497, leading to the status quo. But now there should be no reason not to move that step back to its more logical home in instrumentaion.

Author: matthiaskrgr