Call `FixupRelativePath` when opening output files to ensure that
`-working-directory` is used when checking up front for write failures,
not just when finalizing the files at the end. This also moves the
temporary file into the same directory as the output file.
Reviewed By: benlangmuir
Differential Revision: https://reviews.llvm.org/D95497
On z/OS, the following error message is not matched correctly in lit tests.
```
EDC5129I No such file or directory.
```
This patch uses a lit config substitution to check for platform specific error messages.
Reviewed By: muiez, jhenderson
Differential Revision: https://reviews.llvm.org/D95246
Steven Wu