Replace <experimental/filesystem> includes with <filesystem>

https://bugs.dolphin-emu.org/issues/11770

Readme.md

@@ -39,7 +39,7 @@ Dolphin can only be installed on devices that satisfy the above requirements. At
 ## Building for Windows
 
 Use the solution file `Source/dolphin-emu.sln` to build Dolphin on Windows.
-Visual Studio 2017 is a hard requirement. Other compilers might be
+Visual Studio 2017 15.7 is a hard requirement. Other compilers might be
 able to build Dolphin on Windows but have not been tested and are not
 recommended to be used. Git and Windows 10 SDK 10.0.17134.0 must be installed when building.
 

Source/Core/Common/FileSearch.cpp

@@ -10,8 +10,8 @@
 
 #ifdef _MSC_VER
 #include <Windows.h>
-#include <experimental/filesystem>
-namespace fs = std::experimental::filesystem;
+#include <filesystem>
+namespace fs = std::filesystem;
 #define HAS_STD_FILESYSTEM
 #else
 #include <cstring>

Source/Core/Core/Boot/Boot.cpp

@@ -5,8 +5,8 @@
 #include "Core/Boot/Boot.h"
 
 #ifdef _MSC_VER
-#include <experimental/filesystem>
-namespace fs = std::experimental::filesystem;
+#include <filesystem>
+namespace fs = std::filesystem;
 #define HAS_STD_FILESYSTEM
 #endif
 
@@ -90,7 +90,7 @@ static std::vector<std::string> ReadM3UFile(const std::string& m3u_path,
 #ifdef HAS_STD_FILESYSTEM
       const fs::path path_line = fs::u8path(line);
       const std::string path_to_add =
-          path_line.is_relative() ? fs::u8path(folder_path).append(path_line).u8string() : line;
+          path_line.is_relative() ? (fs::u8path(folder_path) / path_line).u8string() : line;
 #else
       const std::string path_to_add = line.front() != '/' ? folder_path + line : line;
 #endif