Fix cfg warnings in compiletest
Fix cfg warnings in libcoretest
Fix cfg warnings in librustc_back
Remove `virtual` structs from the language
Remove `virtual` struct tests
Remove `virtual` structs from the reference
auto merge of #17949 : ebfe/rust/cfg_warn, r=sfackler

This converts old style cfgs that generate warnings during `make check` to the new syntax.
auto merge of #17952 : jakub-/rust/remove-virtual-structs, r=eddyb

Closes #17861.