Debug info is wrong when rustc expects a function to be inlined but llvm doesn't inline it

| opt-level | inline-threshold | number of errors |
|--------------|-----------------------|--------------------------|
| 0 | 2000 | 5243 |
| 0 | 255 | 3967 |
| 0 | 0 | 3012 |
| 3 | 0 | 309 |
| 3 | default | 346 |
| 3 | 2000 | 255 |

Errors where measured with:
`llvm-dwarfdump -verify | grep error | wc -l`

Reproduce the reported debug-info problem using the opt-level and inline-threshold combinations in the table, then run llvm-dwarfdump -verify | grep error | wc -l to measure it. Trace how rustc and LLVM handle functions expected to be inlined, and consider the issue complete when the verifier errors and the mismatch in inline behavior are explained or corrected.

Written by the indexing model from the issue text.