[LibTooling] Fix dangling references in RangeSelector.
RangeSelector had a number of cases of capturing a StringRef in a lambda, which
lead to dangling references. This change converts all uses in the API of
StringRef to std::string to avoid this problem. std::string in the API is
a reasonable choice, because the combinators are always storing the string
beyond the life of the combinator construction.
Reviewers: ilya-biryukov, gribozavr
Differential Revision: https://reviews.llvm.org/D62328