Port yuzu-emu/yuzu#2511: "common/file_util: Minor cleanup" (#4782)

* common/file_util: Make IOFile's WriteString take a std::string_view

We don't need to force the usage of a std::string here, and can instead
use a std::string_view, which allows writing out other forms of strings
(e.g. C-style strings) without any unnecessary heap allocations.

* common/file_util: Remove unnecessary c_str() calls

The file stream open functions have supported std::string overloads
since C++11, so we don't need to use c_str() here. Same behavior, less
code.

* common/file_util: Make ReadFileToString and WriteStringToFile consistent

Makes the parameter ordering consistent, and also makes the filename
parameter a std::string. A std::string would be constructed anyways with
the previous code, as IOFile's only constructor with a filepath is one
taking a std::string.

We can also make WriteStringToFile's string parameter utilize a
std::string_view for the string, making use of our previous changes to
IOFile.

* common/file_util: Remove duplicated documentation comments

These are already present within the header, so they don't need to be
repeated in the cpp file.

* common/file_util: Make GetCurrentDir() return a std::optional

nullptr was being returned in the error case, which, at a glance may
seem perfectly OK... until you realize that std::string has the
invariant that it may not be constructed from a null pointer. This
means that if this error case was ever hit, then the application would
most likely crash from a thrown exception in std::string's constructor.

Instead, we can change the function to return an optional value,
indicating if a failure occurred.

* common/file_util: Remove unnecessary return at end of void StripTailDirSlashes()

While we're at it, also invert the conditional into a guard clause.

src/common/file_util.h

@@ -9,6 +9,7 @@
 #include <fstream>
 #include <functional>
 #include <limits>
+#include <optional>
 #include <string>
 #include <type_traits>
 #include <vector>
@@ -115,7 +116,7 @@ u64 ScanDirectoryTree(const std::string& directory, FSTEntry& parent_entry,
 bool DeleteDirRecursively(const std::string& directory, unsigned int recursion = 256);
 
 // Returns the current directory
-std::string GetCurrentDir();
+std::optional<std::string> GetCurrentDir();
 
 // Create directory and copy contents (does not overwrite existing files)
 void CopyDir(const std::string& source_path, const std::string& dest_path);
@@ -141,8 +142,9 @@ const std::string& GetExeDirectory();
 std::string AppDataRoamingDirectory();
 #endif
 
-size_t WriteStringToFile(bool text_file, const std::string& str, const char* filename);
-size_t ReadFileToString(bool text_file, const char* filename, std::string& str);
+std::size_t WriteStringToFile(bool text_file, const std::string& filename, std::string_view str);
+
+std::size_t ReadFileToString(bool text_file, const std::string& filename, std::string& str);
 
 /**
  * Splits the filename into 8.3 format
@@ -228,8 +230,8 @@ public:
         return WriteArray(&object, 1);
     }
 
-    std::size_t WriteString(const std::string& str) {
-        return WriteArray(str.c_str(), str.length());
+    std::size_t WriteString(std::string_view str) {
+        return WriteArray(str.data(), str.length());
     }
 
     bool IsOpen() const {
@@ -267,8 +269,8 @@ private:
 template <typename T>
 void OpenFStream(T& fstream, const std::string& filename, std::ios_base::openmode openmode) {
 #ifdef _MSC_VER
-    fstream.open(Common::UTF8ToUTF16W(filename).c_str(), openmode);
+    fstream.open(Common::UTF8ToUTF16W(filename), openmode);
 #else
-    fstream.open(filename.c_str(), openmode);
+    fstream.open(filename, openmode);
 #endif
 }