Revert "tdf#112689: Replace changed O(U)StringBuffer::append() with operator+"

Sorry, but this change breaks the build:
bridges/source/cpp_uno/msvc_win32_x86-64/except.cxx(884): error C2110: '+': cannot add two pointers

This reverts commit 86738479.

bridges/source/cpp_uno/msvc_win32_x86-64/except.cxx

@@ -879,12 +879,15 @@ int mscx_filterCppException(
                     &pExcTD, aUNOname.pData );
                 if (pExcTD == nullptr)
                 {
-                    OUString sMsg = "[mscx_uno bridge error] UNO type of "
-                                  + "C++ exception unknown: \""
-                                  + aUNOname + "\", RTTI-name=\""
-                                  + aRTTIname + "\"!";
-                    RuntimeException exc( sMsg );
-
+                    OUStringBuffer buf;
+                    buf.append(
+                            "[mscx_uno bridge error] UNO type of "
+                            "C++ exception unknown: \"" );
+                    buf.append( aUNOname );
+                    buf.append( "\", RTTI-name=\"" );
+                    buf.append( aRTTIname );
+                    buf.append( "\"!" );
+                    RuntimeException exc( buf.makeStringAndClear() );
                     uno_type_any_constructAndConvert(
                         pUnoExc, &exc,
                         cppu::UnoType<decltype(exc)>::get().getTypeLibType(), pCpp2Uno );