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

This reverts commit bac85ab.

Author: mgehre

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 LLVM_GSL_POINTER StringRef {
   public:
     static const size_t npos = ~size_t(0);
 

llvm/include/llvm/Support/Compiler.h

@@ -289,6 +289,14 @@
 #define LLVM_REQUIRE_CONSTANT_INITIALIZATION
 #endif
 
+/// LLVM_GSL_POINTER - Apply this to non-owning classes like
+/// StringRef to enable lifetime warnings.
+#if LLVM_HAS_CPP_ATTRIBUTE(gsl::Pointer)
+#define LLVM_GSL_POINTER [[gsl::Pointer]]
+#else
+#define LLVM_GSL_POINTER
+#endif
+
 /// LLVM_EXTENSION - Support compilers where we have a keyword to suppress
 /// pedantic diagnostics.
 #ifdef __GNUC__