Move [[gsl::Pointer]] into llvm/include/llvm/Support/Compiler.h

[mgehre]

Summary

Make [[gsl::Pointer]] conditional on compiler support

[mgehre]

Fix typo

[RKSimon]

I can confirm this works for MSVC builds

[aaron.ballman]

LGTM!

[RKSimon]

Confirmed on ubuntu gcc7.5 as well - LGTM, cheers.

[mgehre]

Committed as 6d2f73f821ed5ea584869924b150ac2e6e65c12e

llvm/include/llvm/ADT/StringRef.h

namespace llvm {
 
   /// StringRef - Represent a constant reference to a string, i.e. a character
   /// array and a length, which need not be null terminated.
   ///
   /// This class does not own the string data, it is expected to be used in
   /// 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 [[gsl::Pointer]] StringRef {
+  class LLVM_GSL_POINTER StringRef {
   public:
     static const size_t npos = ~size_t(0);
 
     using iterator = const char *;
     using const_iterator = const char *;
     using size_type = size_t;
 
   private:

llvm/include/llvm/Support/Compiler.h

 /// they are constant initialized.
 #if LLVM_HAS_CPP_ATTRIBUTE(clang::require_constant_initialization)
 #define LLVM_REQUIRE_CONSTANT_INITIALIZATION                                   \
   [[clang::require_constant_initialization]]
 #else
 #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__
 #define LLVM_EXTENSION __extension__
 #else
 #define LLVM_EXTENSION
 #endif