Add support to specify substitutions externally

[MichalStrehovsky]

ILLinker allows specifying substitutions on the command line, same as descriptors. This can be useful for various workarounds. Add support for this for ILC as well.

Cc @dotnet/ilc-contrib

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Issue Details

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ILLinker allows specifying substitutions on the command line, same as descriptors. This can be useful for various workarounds. Add support for this for ILC as well.

Cc @dotnet/ilc-contrib

Author:	MichalStrehovsky
Assignees:	-
Labels:	area-NativeAOT-coreclr
Milestone:	-

[vitek-karas]

The changes look good.

There are some illink tests which are currently disabled for NativeAOT because of this. I think it should be possible to reenable them (with some more infra work):
https://github.com/dotnet/runtime/blob/main/src/tools/illink/test/Mono.Linker.Tests.Cases/Warnings/CanWarnAsError.cs

Feel free to create an issue for that and link it to #82447. Doesn't have to be done now or as part of this PR.

[MichalStrehovsky]

The changes look good.

There are some illink tests which are currently disabled for NativeAOT because of this. I think it should be possible to reenable them (with some more infra work): https://github.com/dotnet/runtime/blob/main/src/tools/illink/test/Mono.Linker.Tests.Cases/Warnings/CanWarnAsError.cs

Feel free to create an issue for that and link it to #82447. Doesn't have to be done now or as part of this PR.

Thanks! Opened #88647 and linked from the uber issue.