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/utils_tests.cpp

@@ -14,6 +14,13 @@
 //
 ///////////////////////////////////////////////////////////////////////////////
 
+#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 : 26440 26426) // from catch
+
+#endif
+
 #include <catch/catch.hpp> // for AssertionHandler, StringRef, CHECK, TEST_...
 
 #include <gsl/gsl_util> // for narrow, finally, narrow_cast, narrowing_e...
@@ -85,6 +92,7 @@ TEST_CASE("finally_function_ptr")
     CHECK(j == 1);
 }
 
+GSL_SUPPRESS(con.4) // NO-FORMAT: attribute
 TEST_CASE("narrow_cast")
 {
     int n = 120;
@@ -96,6 +104,7 @@ TEST_CASE("narrow_cast")
     CHECK(uc == 44);
 }
 
+GSL_SUPPRESS(con.5) // NO-FORMAT: attribute
 TEST_CASE("narrow")
 {
     int n = 120;