Minor: fix msvc warning "not all control paths return a value" (#5650)

When building with /Od - default cmake debug build for me, the
__assume(false); trick does not work to get rid of the C4714 warnings

https://godbolt.org/z/a6xGnfP7d

D:\tmp\cppcheck\lib\keywords.cpp(205): warning C4715:
'Keywords::getOnly': not all control paths return a value
D:\tmp\cppcheck\lib\keywords.cpp(226): warning C4715:
'Keywords::getOnly': not all control paths return a value
D:\tmp\cppcheck\lib\keywords.cpp(168): warning C4715:
'Keywords::getAll': not all control paths return a value
D:\tmp\cppcheck\lib\keywords.cpp(188): warning C4715:
'Keywords::getAll': not all control paths return a value

Proposed fix: also define NORETURN to [[noreturn]] when according to
__has_cpp_attribute [[noreturn]] is supported
https://en.cppreference.com/w/cpp/feature_test

(For previous discussion see also
#5497)

lib/config.h

@@ -51,14 +51,21 @@
 #endif
 
 // C++11 noreturn
-#if (defined(__GNUC__) && (__GNUC__ >= 5)) \
+#if defined __has_cpp_attribute
+#  if __has_cpp_attribute (noreturn)
+#    define NORETURN [[noreturn]]
+#  endif
+#endif
+#if !defined(NORETURN)
+#  if (defined(__GNUC__) && (__GNUC__ >= 5)) \
     || defined(__clang__) \
     || defined(__CPPCHECK__)
-#  define NORETURN [[noreturn]]
-#elif defined(__GNUC__)
-#  define NORETURN __attribute__((noreturn))
-#else
-#  define NORETURN
+#    define NORETURN [[noreturn]]
+#  elif defined(__GNUC__)
+#    define NORETURN __attribute__((noreturn))
+#  else
+#    define NORETURN
+#  endif
 #endif
 
 // fallthrough