ADT: Surface LowerCase argument for utohexstr

The underlying function. utohex_buffer, already supports an argument for
deciding if the hex characters should be upper or lower case.  Expose an
identical argument for utohexstr.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212991 91177308-0d34-0410-b5e6-96231b3b80d8