Fix the build.
[xbuild]: Allow expressions inside a dynamic <ItemGroup>'s metadata.

Fixes #14661.
[xbuild]: Provide diagnostic logging of all executed targets.
[xbuild]: Check a <Target>'s "Returns" first, then fall-back to "Outputs".

This fixes a problem with the "Microsoft.Bcl.Build" NuGet package, which
overrides the "GetTargetPath" target in a custom .targets file and uses
"Returns" instead of "Outputs".
[sgen] Link unit test programs with all libs.

Should fix the build for monodroid.
[corlib] Update CurrentRegion to work with DefaultThreadCurrentCulture
Remove some more mdb bits.