Auto merge of #148350 - scottmcm:slice-range-attributes, r=JonathanBrouwer,davidtwco

Add LLVM range attributes to slice length parameters

It'll take a bunch more work to do this in layout -- because of cycles in `struct Foo<'a>(&'a Foo<'a>);` -- so until we figure out how to do that well, just look for slices specifically and add the proper range for the length.

Author: bors