fix cppcorecheck warnings (#703)

* Added c++17 test configurations for clang5.0 and clang6.0

* Fixed CppCoreCheck warnings in GSL and tests

- Added CMakeSettings.json for VS Open Folder configuration
	- So we can easily run CppCoreCheck in VS
- Fixed CppCorecheck warnings where it made sense
- Suppressed the rest
	- Some suppression does not work due to compiler/tool bugs,
          so replaced by #pragma disable
	- CppCoreCheck has noise, suppressed those with comments
	- Catch produces many warnings, blanket-supressed them all
- Had to fix clang formatting to keep attributes in place
	- clang-format does not support attributes, so I am using
        - "CommentPragmas: '^ NO-FORMAT:'" to skip formatiting on them
- Removed GSL_NOEXCEPT macro, removed incorred noexcepts

* Ignore unknown attributes

* ignore unknown attributes in noexception mode tests

* fixed C26472 in at()

* created GSL_SUPPRESS macro to allow all compilers to parse suppression attributes

* try to fix gcc compilation problems with attributes

* ignore gsl::suppress for gcc

* move suppression to function level on return statements

clang5.0 and up does not allow attributes on return statemets in constexpr functions

* move suppression to function level on return statements

* use GSL_SUPPRESS in algorithm_tests

* Addressed PR comments

include/gsl/gsl_assert

@@ -20,6 +20,20 @@
 #include <exception>
 #include <stdexcept> // for logic_error
 
+//
+// make suppress attributes parse for some compilers
+// Hopefully temporary until suppresion stardization occurs
+//
+#if defined (_MSC_VER)
+#define GSL_SUPPRESS(x) [[gsl::suppress(x)]]
+#else
+#if defined(__clang__)
+#define GSL_SUPPRESS(x) [[gsl::suppress("x")]]
+#else
+#define GSL_SUPPRESS(x)
+#endif // __clang__
+#endif // _MSC_VER
+
 //
 // Temporary until MSVC STL supports no-exceptions mode.
 // Currently terminate is a no-op in this mode, so we add termination behavior back
@@ -85,6 +99,7 @@ namespace details
 
     typedef void  (__cdecl *terminate_handler)();
     
+    GSL_SUPPRESS(f.6) // NO-FORMAT: attribute
     [[noreturn]] inline void __cdecl default_terminate_handler()
     {
         __fastfail(RANGE_CHECKS_FAILURE);
@@ -110,11 +125,7 @@ namespace details
 #if defined(GSL_TERMINATE_ON_CONTRACT_VIOLATION)
 
     template <typename Exception>
-#if defined(__clang__) || defined(__GNUC__)
-#else
-    [[gsl::suppress(f.6)]]
-#endif
-    [[noreturn]] void throw_exception(Exception&&)
+    [[noreturn]] void throw_exception(Exception&&) noexcept
     {
         gsl::details::terminate();
     }