Fix windows libc++ fs::path fstream compile errors

As reported by hebasto in https://github.com/bitcoin/bitcoin/issues/33545,
newer libc++ versions implementing https://wg21.link/lwg3430 will no longer
implicitly convert `fs::path` objects to `std::filesystem::path` objects when
constructing `std::ifstream` and `std::ofstream` types.

This is not a problem in Unix systems since `fs::path` objects use
`std::string` as their native string type, but it causes compile errors on
Windows which use `std::wstring` as their string type, since `fstream`s can't
be constructed from `wstring`s.

Fix the windows libc++ compile errors by adding a new `fs::path::std_path()`
method and using it construct `fstream`s more portably.

Additionally, delete `fs::path`'s implicit `native_string` conversion so these
errors will not go undetected in the future, even though there is not currently
a CI job testing Windows libc++ builds.

src/qt/guiutil.cpp

@@ -447,7 +447,7 @@ bool openBitcoinConf()
     fs::path pathConfig = gArgs.GetConfigFilePath();
 
     /* Create the file */
-    std::ofstream configFile{pathConfig, std::ios_base::app};
+    std::ofstream configFile{pathConfig.std_path(), std::ios_base::app};
 
     if (!configFile.good())
         return false;
@@ -607,7 +607,7 @@ fs::path static GetAutostartFilePath()
 
 bool GetStartOnSystemStartup()
 {
-    std::ifstream optionFile{GetAutostartFilePath()};
+    std::ifstream optionFile{GetAutostartFilePath().std_path()};
     if (!optionFile.good())
         return false;
     // Scan through file for "Hidden=true":
@@ -639,7 +639,7 @@ bool SetStartOnSystemStartup(bool fAutoStart)
 
         fs::create_directories(GetAutostartDir());
 
-        std::ofstream optionFile{GetAutostartFilePath(), std::ios_base::out | std::ios_base::trunc};
+        std::ofstream optionFile{GetAutostartFilePath().std_path(), std::ios_base::out | std::ios_base::trunc};
         if (!optionFile.good())
             return false;
         ChainType chain = gArgs.GetChainType();

src/qt/test/optiontests.cpp

@@ -70,7 +70,7 @@ void OptionTests::migrateSettings()
     QVERIFY(!settings.contains("fUseSeparateProxyTor"));
     QVERIFY(!settings.contains("addrSeparateProxyTor"));
 
-    std::ifstream file(gArgs.GetDataDirNet() / "settings.json");
+    std::ifstream file((gArgs.GetDataDirNet() / "settings.json").std_path());
     std::string default_warning = strprintf("This file is automatically generated and updated by %s. Please do not edit this file while the node "
                                             "is running, as any changes might be ignored or overwritten.",
                                             CLIENT_NAME);