Remove into_vec method from &[T]

[breaking-change]
Closes #17916
Fix cfg warnings in compiletest
Fix cfg warnings in libcoretest
Fix cfg warnings in librustc_back
auto merge of #17942 : JIghtuse/rust/master, r=alexcrichton

[breaking-change]
Closes #17916
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.