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

tests/test.cpp

@@ -15,4 +15,12 @@
 ///////////////////////////////////////////////////////////////////////////////
 
 #define CATCH_CONFIG_MAIN
+
+#ifdef _MSC_VER
+
+// blanket turn off warnings from CppCoreCheck from catch
+// so people aren't annoyed by them when running the tool.
+#pragma warning(disable : 26401 26409 26415 26418 26426 26429 26432 26433 26434 26435 26436 26439 26440 26443 26444 26446 26447 26451 26460 26461 26466 26472 26481 26482 26485 26492 26493 26494 26495 26496 26497) // from catch
+#endif // _MSC_VER
+
 #include <catch/catch.hpp>