Add a StringLexer utility class that can be used when you have string data that needs to be parsed - I don't think such a general purpose facility is part of LLVM, and I am going to need this, so just add it to lldb_utility

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@215133 91177308-0d34-0410-b5e6-96231b3b80d8