Finish impl of alternate pins.

It appears this got shelved while waiting for the sexpr format to be
firmed up, and then I forgot about it.

Fixes https://gitlab.com/kicad/code/kicad/issues/6189

eeschema/sch_pin.cpp

@@ -37,6 +37,21 @@ SCH_PIN::SCH_PIN( LIB_PIN* aLibPin, SCH_COMPONENT* aParentComponent ) :
 }
 
 
+/**
+ * Create a proxy pin from an alternate pin designation.
+ * The LIB_PIN data will be filled in when the pin is resolved (see SCH_COMPONENT::UpdatePins).
+ */
+SCH_PIN::SCH_PIN( SCH_COMPONENT* aParentComponent, const wxString& aNumber,
+                  const wxString& aAlt ) :
+    SCH_ITEM( aParentComponent, SCH_PIN_T )
+{
+    m_alt = aAlt;
+    m_number = aNumber;
+    m_libPin = nullptr;
+    m_isDangling = true;
+}
+
+
 SCH_PIN::SCH_PIN( const SCH_PIN& aPin ) :
         SCH_ITEM( aPin )
 {

eeschema/sch_pin.h

@@ -48,6 +48,8 @@ class SCH_PIN : public SCH_ITEM
 public:
     SCH_PIN( LIB_PIN* aLibPin, SCH_COMPONENT* aParentComponent );
 
+    SCH_PIN( SCH_COMPONENT* aParentComponent, const wxString& aNumber, const wxString& aAlt );
+
     SCH_PIN( const SCH_PIN& aPin );
 
     SCH_PIN& operator=( const SCH_PIN& aPin );

eeschema/sch_plugins/kicad/sch_sexpr_parser.cpp

@@ -2286,17 +2286,24 @@ SCH_COMPONENT* SCH_SEXPR_PARSER::parseSchematicSymbol()
 
         case T_pin:
         {
-            SCH_PIN* pin = new SCH_PIN( nullptr, symbol.get() );
+            // Read an alternate pin designation
+            wxString number;
+            wxString alt;
 
             NeedSYMBOL();
-            pin->SetNumber( FromUTF8() );
+            number = FromUTF8();
+
+            token = NextTok();
+
+            if( token != T_LEFT )
+                Expecting( T_LEFT );
 
             token = NextTok();
 
             if( token == T_alternate )
             {
                 NeedSYMBOL();
-                pin->SetAlt( FromUTF8() );
+                alt = FromUTF8();
                 NeedRIGHT();
             }
             else
@@ -2304,7 +2311,10 @@ SCH_COMPONENT* SCH_SEXPR_PARSER::parseSchematicSymbol()
                 Expecting( "alternate" );
             }
 
-            symbol->GetPins().push_back( pin );
+            // Create a proxy pin to hold the alternate designation until the parent
+            // component resolves its pins.
+            symbol->GetRawPins().emplace_back( std::make_unique<SCH_PIN>( symbol.get(),
+                                                                          number, alt ) );
 
             NeedRIGHT();
         }