[C++11] Make this interface accept const Use pointers and use override

[C++11] Make this interface accept const Use pointers and use override
to ensure we don't mess up any of the overrides. Necessary for cleaning
up the Value use iterators and enabling range-based traversing of use
lists.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202958 91177308-0d34-0410-b5e6-96231b3b80d8