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/no_exception_ensure_tests.cpp

@@ -17,10 +17,10 @@
 #include <cstdlib>      // for std::exit
 #include <gsl/span>     // for span
 
-int operator_subscript_no_throw()
+int operator_subscript_no_throw() noexcept
 {
     int arr[10]; 
-    gsl::span<int> sp { arr };
+    const gsl::span<int> sp { arr };
     return sp[11];
 }
 
@@ -30,7 +30,7 @@ void test_terminate()
     std::exit(0);
 }
 
-void setup_termination_handler()
+void setup_termination_handler() noexcept
 {
 #if defined(_MSC_VER)
 
@@ -45,7 +45,7 @@ void setup_termination_handler()
 }
 
 
-int main()
+int main() noexcept
 {
     setup_termination_handler();
     operator_subscript_no_throw();