Update C++ standard to C++20

Closes #6945

Author: expipiplus1

cmake/CompilerFlags.cmake

@@ -175,7 +175,7 @@ function(set_default_compile_options target)
             ON
             # C++ standard
             CXX_STANDARD
-            17
+            20
             # pic
             POSITION_INDEPENDENT_CODE
             ON

source/core/slang-io.cpp

@@ -752,7 +752,7 @@ String Path::getRelativePath(String base, String path)
     auto result = std::filesystem::relative(p2, p1, ec);
     if (ec)
         return path;
-    return String(UnownedStringSlice(result.generic_u8string().c_str()));
+    return String(reinterpret_cast<const char*>(result.generic_u8string().c_str()));
 }
 
 SlangResult Path::remove(const String& path)

tools/slang-test/slang-test-main.cpp

@@ -58,7 +58,7 @@ extern "C"
 
 extern "C"
 {
-    __declspec(dllexport) extern const char* D3D12SDKPath = u8".\\D3D12\\";
+    __declspec(dllexport) extern const char* D3D12SDKPath = ".\\D3D12\\";
 }
 #endif
 

tools/test-server/test-server-main.cpp

@@ -28,7 +28,7 @@ extern "C"
 
 extern "C"
 {
-    __declspec(dllexport) extern const char* D3D12SDKPath = u8".\\D3D12\\";
+    __declspec(dllexport) extern const char* D3D12SDKPath = ".\\D3D12\\";
 }
 #endif