[PDB] Attempt fix for debug-info-codeview-buildinfo.c test

This is a bit a shot in the dark, as it doesn't occur on my Windows 10 machines, nor on x64 Linux Ubuntu 18.04.
This patch tries to fix the following kind of error:
- http://lab.llvm.org:8011/builders/clang-ppc64le-linux/builds/31511/steps/cmake%20stage%201/logs/stdio
- http://lab.llvm.org:8011/builders/clang-ppc64le-linux-lnt/builds/25150/steps/ninja%20check%201/logs/FAIL%3A%20Clang%3A%3Adebug-info-codeview-buildinfo.c
- http://lab.llvm.org:8011/builders/fuchsia-x86_64-linux/builds/7947/steps/check/logs/stdio