Use wxWidgets conversions to/from UTF-8 in Scintilla code

Scintilla conversions use UTF-16, as indicated by their names, while wxString
uses UTF-32 under Unix, so they don't work correctly for the characters
outside of BMP.

Simply use our own UTF-8 conversions, it doesn't seem possible to get invalid
UTF-8 sequences here anyhow, so the comment saying that Scintilla functions
are used to avoid asserts doesn't seem to be relevant.

Closes #15621.

src/stc/PlatWX.cpp

@@ -1618,44 +1618,25 @@ double ElapsedTime::Duration(bool reset) {
 
 #if wxUSE_UNICODE
 
-#include "UniConversion.h"
+// For historical reasons, we use Scintilla-specific conversion functions, we
-
+// should probably just call FromUTF8()/utf8_str() directly instead now.
-// Convert using Scintilla's functions instead of wx's, Scintilla's are more
-// forgiving and won't assert...
 
 wxString stc2wx(const char* str, size_t len)
 {
-    if (!len)
+    return wxString::FromUTF8(str, len);
-        return wxEmptyString;
-
-    size_t wclen = UTF16Length(str, len);
-    wxWCharBuffer buffer(wclen+1);
-
-    size_t actualLen = UTF16FromUTF8(str, len, buffer.data(), wclen+1);
-    return wxString(buffer.data(), actualLen);
 }
 
 
 
 wxString stc2wx(const char* str)
 {
-    return stc2wx(str, strlen(str));
+    return wxString::FromUTF8(str);
 }
 
 
 wxWX2MBbuf wx2stc(const wxString& str)
 {
-    const wchar_t* wcstr = str.c_str();
+    return str.utf8_str();
-    size_t wclen         = str.length();
-    size_t len           = UTF8Length(wcstr, wclen);
-
-    // The buffer object adds extra byte for the terminating NUL and we must
-    // pass the total length, including this NUL, to UTF8FromUTF16() to ensure
-    // that it NULL-terminates the string.
-    wxCharBuffer buffer(len);
-    UTF8FromUTF16(wcstr, wclen, buffer.data(), len + 1);
-
-    return buffer;
 }
 
 #endif