[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

llvm/include/llvm/ADT/StringRef.h

@@ -54,7 +54,7 @@ namespace llvm {
   /// situations where the character data resides in some other buffer, whose
   /// lifetime extends past that of the StringRef. For this reason, it is not in
   /// general safe to store a StringRef.
-  class StringRef {
+  class [[gsl::Pointer]] StringRef {
   public:
     static const size_t npos = ~size_t(0);