Preserve original PDB path in Debug header

[sbomer]

When writing debug symbols, Cecil places them into a .pdb file adjacent to the .dll in the output. It also uses the path of this output .pdb as the path to the PDB in the .dll's CodeView Debug Directory entry, which breaks scenarios using PathMap.

This change fixes the PDB path in the CodeView Debug Directory entry to preserve the original path if it was present in the input .dll (but doesn't otherwise affect where Cecil looks for or writes out .pdb files).

Fixes dotnet/runtime#99594.

[jtschuster]

Thank you!

[sbomer]

@vitek-karas pointed out that when not using deterministic source paths, this would break because it would leave the output assembly pointing to the original pdb instead of the new pdb. So this will take more work (it may need to be conditioned on some new flag to control the behavior, or detect whether the compilation was done with deterministic source paths).

[sbomer]

I found some more context that indicates the supported way to do this is via a custom symbol writer - so I'll close this and implement a solution in linker.

• Discussion around full path, copying input path, using just filename, etc: Restored full path in filename property jbevain/cecil#554
• Xamarin-macios solution to a similar problem: rolfbjarne/xamarin-macios@53874c8