[ADT] Support converting to lowercase string in toHex

This is useful in certain use-cases such as D51833.

Differential Revision: https://reviews.llvm.org/D51835

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@341852 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed