[LifetimeAnalysis] Add [[gsl::Pointer]] to llvm::StringRef

Summary:
This detected the bugs fixed in
  https://reviews.llvm.org/D66442
and
  https://reviews.llvm.org/D66440

The warning itself was implemented in
  https://reviews.llvm.org/D63954
  https://reviews.llvm.org/D64256
  https://reviews.llvm.org/D65120
  https://reviews.llvm.org/D65127
  https://reviews.llvm.org/D66152

Reviewers: zturner, mehdi_amini, gribozavr

Subscribers: dexonsmith, Szelethus, xazax.hun, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66443