Suppress some noisy / buggy warnings (#1136)

Two warnings were being emitted in the MSVC+LLVM tests. 

The warning `-Wunsafe-buffer-usage` is initially introduced in some capacity here https://reviews.llvm.org/D137346 pointing to documentation at https://discourse.llvm.org/t/rfc-c-buffer-hardening/65734. The warning is a stylistic checker whose goal is to "emit a warning every time an unsafe operation is performed on a raw pointer". This type of programming model is not useful for library implementations of types such as `span`, where direct manipulation of raw pointers is inevitable, so disable the warning altogether.

There is also a false-positive warning llvm/llvm-project#65689 that I've disabled inline.

include/gsl/span

@@ -777,8 +777,15 @@ span(const Container&) -> span<Element>;
 #endif // ( defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201611L) )
 
 #if defined(GSL_USE_STATIC_CONSTEXPR_WORKAROUND)
+#if defined(__clang__) && defined(_MSC_VER) && defined(__cplusplus) && (__cplusplus < 201703L)
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wdeprecated" // Bug in clang-cl.exe which raises a C++17 -Wdeprecated warning about this static constexpr workaround in C++14 mode.
+#endif // defined(__clang__) && defined(_MSC_VER) && defined(__cplusplus) && (__cplusplus < 201703L)
 template <class ElementType, std::size_t Extent>
 constexpr const typename span<ElementType, Extent>::size_type span<ElementType, Extent>::extent;
+#if defined(__clang__) && defined(_MSC_VER) && defined(__cplusplus) && (__cplusplus < 201703L)
+#pragma clang diagnostic pop
+#endif // defined(__clang__) && defined(_MSC_VER) && defined(__cplusplus) && (__cplusplus < 201703L)
 #endif
 
 namespace details

tests/CMakeLists.txt

@@ -121,7 +121,12 @@ if(MSVC) # MSVC or simulating MSVC
     )
     check_cxx_compiler_flag("-Wno-reserved-identifier" WARN_RESERVED_ID)
     if (WARN_RESERVED_ID)
-        target_compile_options(gsl_tests_config INTERFACE "-Wno-reserved-identifier")
+      target_compile_options(gsl_tests_config INTERFACE "-Wno-reserved-identifier")
+    endif()
+    check_cxx_compiler_flag("-Wno-unsafe-buffer-usage" WARN_UNSAFE_BUFFER)
+    if (WARN_UNSAFE_BUFFER)
+      # This test uses very greedy heuristics such as "no pointer arithmetic on raw buffer"
+      target_compile_options(gsl_tests_config INTERFACE "-Wno-unsafe-buffer-usage")
     endif()
 else()
     target_compile_options(gsl_tests_config INTERFACE
@@ -255,7 +260,12 @@ if(MSVC) # MSVC or simulating MSVC
     )
     check_cxx_compiler_flag("-Wno-reserved-identifier" WARN_RESERVED_ID)
     if (WARN_RESERVED_ID)
-        target_compile_options(gsl_tests_config_noexcept INTERFACE "-Wno-reserved-identifier")
+      target_compile_options(gsl_tests_config_noexcept INTERFACE "-Wno-reserved-identifier")
+    endif()
+    check_cxx_compiler_flag("-Wno-unsafe-buffer-usage" WARN_UNSAFE_BUFFER)
+    if (WARN_UNSAFE_BUFFER)
+      # This test uses very greedy heuristics such as "no pointer arithmetic on raw buffer"
+      target_compile_options(gsl_tests_config_noexcept INTERFACE "-Wno-unsafe-buffer-usage")
     endif()
 else()
     target_compile_options(gsl_tests_config_noexcept INTERFACE