Use __func__ instead of __FUNCTION__ where applicable

This replaces usages of the non-standard __FUNCTION__ macro with the standard
mandated __func__ identifier.

__FUNCTION__ is a preprocessor definition that is provided as an
extension by compilers. This was the only convenient option to rely on
pre-C++11. However, C++11 and greater mandate the predefined identifier
__func__, which lets us accomplish the same thing.

The difference between the two, however, is that __func__ isn't a
preprocessor macro, it's an actual identifier that exists at function
scope. The C++17 draft standard (N4659) at section [dcl.fct.def.general]
paragraph 8 states:

"
The function-local predefined variable __func__ is defined as if a
definition of the form

static const char __func__[] = "function-name ";

had been provided, where function-name is an implementation-defined
string. It is unspecified whether such
a variable has an address distinct from that of any other object in the
program.
"

Thankfully, we don't do any macro or string concatenation with __FUNCTION__
that can't be modified to use __func__.

Source/Core/VideoBackends/Vulkan/VulkanLoader.h

@@ -40,6 +40,6 @@ void UnloadVulkanLibrary();
 const char* VkResultToString(VkResult res);
 void LogVulkanResult(int level, const char* func_name, VkResult res, const char* msg, ...);
 
-#define LOG_VULKAN_ERROR(res, ...) LogVulkanResult(2, __FUNCTION__, res, __VA_ARGS__)
+#define LOG_VULKAN_ERROR(res, ...) LogVulkanResult(2, __func__, res, __VA_ARGS__)
 
 }  // namespace Vulkan