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_algorithm

@@ -31,15 +31,12 @@
 #pragma warning(disable : 4127) // conditional expression is constant
 #pragma warning(disable : 4996) // unsafe use of std::copy_n
 
-// blanket turn off warnings from CppCoreCheck for now
-// so people aren't annoyed by them when running the tool.
-// more targeted suppressions will be added in a future update to the GSL
-#pragma warning(disable : 26481 26482 26483 26485 26490 26491 26492 26493 26495)
 #endif // _MSC_VER
 
 namespace gsl
 {
-
+// Note: this will generate faster code than std::copy using span iterator in older msvc+stl
+// not necessary for msvc since VS2017 15.8 (_MSC_VER >= 1915)
 template <class SrcElementType, std::ptrdiff_t SrcExtent, class DestElementType,
           std::ptrdiff_t DestExtent>
 void copy(span<SrcElementType, SrcExtent> src, span<DestElementType, DestExtent> dest)
@@ -51,6 +48,7 @@ void copy(span<SrcElementType, SrcExtent> src, span<DestElementType, DestExtent>
                   "Source range is longer than target range");
 
     Expects(dest.size() >= src.size());
+    GSL_SUPPRESS(stl.1) // NO-FORMAT: attribute
     std::copy_n(src.data(), src.size(), dest.data());
 }