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.
Guide: develop the exposition of arrays, vectors, and slices.

The array is the fundamental concept; vectors are growable arrays, and
slices are views into either.  Show common array ops up front: length
and iteration.  Mention arrays are immutable by default.  Highlight
definite initialization and bounds-checking as safety features.  Show
that you only need a type suffix on one element of initializers.
Explain that vectors are a value-add library type over arrays, not a
fundamental type; show they have the same "interface." Motivate slices
as efficient views into arrays; explain you can slice vectors, Strings,
&str because they're backed by arrays.