StringLiteral: intellisense workaround

[lawrence_danna]

Summary

The intellisense parser from Visual Studio Code will pretend to be
clang by defining __clang__, but it doesn't fully implement enable_if.
It does define __INTELLISENSE__ however, so we can avoid spurious
errors by also checking for that.

https://github.com/microsoft/vscode-cpptools/issues/4246

[JDevlieghere]

Although this makes sense as a local workaround I'm not convinced this is worth checking in.

[lawrence_danna]

ok

llvm/include/llvm/ADT/StringRef.h

#endif
   class StringLiteral : public StringRef {
   private:
     constexpr StringLiteral(const char *Str, size_t N) : StringRef(Str, N) {
     }
 
   public:
     template <size_t N>
     constexpr StringLiteral(const char (&Str)[N])
-#if defined(__clang__) && __has_attribute(enable_if)
+#if !__INTELLISENSE__ && defined(__clang__) && __has_attribute(enable_if)
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wgcc-compat"
         __attribute((enable_if(__builtin_strlen(Str) == N - 1,
                                "invalid string literal")))
 #pragma clang diagnostic pop
 #endif
         : StringRef(Str, N - 1) {
     }