[LibTooling] Fix dangling references in RangeSelector.

Summary:
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

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62328

llvm-svn: 361514