Enable linker analysis and add baselines for warnings

[eerhardt]

dotnet/linker#1030 will change the ILLinker that it will start adding warnings by default when it detects unsafe use of reflection. This will cause our libraries build to start emitting a lot of warnings (in the hundreds).

To keep our build clean, we should add a "baseline" of linker warnings to the repo, which will suppress the warnings. In the future, as we annotate the usages of reflection to make them linker-safe, we can remove the suppressions.

To suppress individual warnings, we can create a LinkerSuppressions.cs file in each project that will contain UnconditionalSuppressMessage attributes. @mateoatr is working on a mechanism that can generate these suppress message attributes that we can use to bootstrap the baselines.

cc @vitek-karas

[joperezr]

@eerhardt @layomia isn't this done already?

[layomia]

@joperezr not yet - I need to update #40691 following the new linker warning codes brought in with #40865.

[layomia]

Moving to 6.0 as this isn't needed for 5.0.

[eerhardt]

Resolved by #40691. Closing.