gsl::at behavior change regarding gsl::span (#985)

* move span specialization of 'at' to <gsl/span> and update the parameter to be taken by reference

* undid previous changes and acted upon decisions made in maintainer sync. Fixed tests failing in /kernel mode

* ran clang-format on the include folder

* ran clang-format on the test folder

Co-authored-by: Jordan Maples <jordan.maples@microsoft.com>

tests/deathTestCommon.h

@@ -0,0 +1,11 @@
+#pragma once
+#include <gsl/assert>
+
+constexpr char deathstring[] = "Expected Death";
+constexpr char failed_set_terminate_deathstring[] = ".*";
+
+// This prevents a failed call to set_terminate from failing the test suite.
+constexpr const char* GetExpectedDeathString(std::terminate_handler handle)
+{
+    return handle ? deathstring : failed_set_terminate_deathstring;
+}