Make annotation case-insensitive

U1 and u1 are allowed but if they both exist in the same schematic, they
will throw an ERC and annotation error.  Reannotating will make them U1
and u2.

Fixes https://gitlab.com/kicad/code/kicad/issues/11862

eeschema/annotate.cpp

@@ -49,10 +49,7 @@ void SCH_EDIT_FRAME::mapExistingAnnotation( std::map<wxString, wxString>& aMap )
 
         curr_full_uuid.push_back( symbol->m_Uuid );
 
-        wxString ref = symbol->GetRef( curr_sheetpath );
-
-        if( symbol->GetUnitCount() > 1 )
-            ref << LIB_SYMBOL::SubReference( symbol->GetUnitSelection( curr_sheetpath ) );
+        wxString ref = symbol->GetRef( curr_sheetpath, true );
 
         if( symbol->IsAnnotated( curr_sheetpath ) )
             aMap[ curr_full_uuid.AsString() ] = ref;

eeschema/sch_reference_list.h

@@ -141,7 +141,7 @@ public:
 
     int CompareRef( const SCH_REFERENCE& item ) const
     {
-        return m_ref.compare( item.m_ref );
+        return m_ref.CmpNoCase( item.m_ref );
     }
 
     int CompareLibName( const SCH_REFERENCE& item ) const
@@ -175,7 +175,7 @@ private:
     friend class SCH_REFERENCE_LIST;
 
     /// Symbol reference prefix, without number (for IC1, this is IC) )
-    UTF8            m_ref;               // it's private, use the accessors please
+    wxString        m_ref;               // it's private, use the accessors please
     SCH_SYMBOL*     m_rootSymbol;        ///< The symbol associated the reference object.
     LIB_SYMBOL*     m_libPart;           ///< The source symbol from a library.
     VECTOR2I        m_symbolPos;         ///< The physical position of the symbol in schematic
@@ -486,7 +486,7 @@ public:
 
             printf( " [%-2d] ref:%-8s num:%-3d lib_part:%s\n",
                     i,
-                    schref.m_ref.c_str(),
+                    schref.m_ref.ToStdString().c_str(),
                     schref.m_numRef,
                     TO_UTF8( schref.GetLibPart()->GetName() ) );
         }