Rollup merge of #129686 - Zalathar:source-region, r=compiler-errors

coverage: Rename `CodeRegion` to `SourceRegion`

LLVM uses the word "code" to refer to a particular kind of coverage mapping. This unrelated usage of the word is confusing, and makes it harder to introduce types whose names correspond to the LLVM classification of coverage kinds.

No functional changes.

Author: matthiaskrgr