Set Anattrib DataDescriptor for local variable references

We now generate FormatDescriptors with WeakSymbolRefs for direct
page references that match variable table entries.

LocalVariableTable got a rewrite.  We need to be unique in both
name and address, but for the address we have to take the width into
account as well.  We also want to sort the display by address
rather than name.  (Some people might want it sorted by name, but
we can worry about that some other time.)

Updated the DefSymbol editor to require value uniqueness.  Note
addresses and constants exist in separate namespaces.

The various symbols are added to the SymbolTable so that uniqueness
checks work correctly.  This also allows the operand generation to
appear to work, but it doesn't yet handle redefinition of symbols.

SourceGen/ProjectFile.cs

@@ -322,10 +322,10 @@ namespace SourceGen {
 
             public SerLocalVariableTable() { }
             public SerLocalVariableTable(LocalVariableTable varTab) {
-                Variables = new List<SerDefSymbol>(varTab.Variables.Count);
-                foreach (KeyValuePair<string, DefSymbol> kvp in varTab.Variables) {
-                    // Note kvp.Key is redundant -- same as kvp.Value.Label
-                    Variables.Add(new SerDefSymbol(kvp.Value));
+                Variables = new List<SerDefSymbol>(varTab.Count);
+                for (int i = 0; i < varTab.Count; i++) {
+                    DefSymbol defSym = varTab[i];
+                    Variables.Add(new SerDefSymbol(defSym));
                 }
 
                 ClearPrevious = varTab.ClearPrevious;
@@ -831,7 +831,13 @@ namespace SourceGen {
                 if (!CreateDefSymbol(serDef, contentVersion, report, out DefSymbol defSym)) {
                     return false;
                 }
-                outLvt.Variables.Add(defSym.Label, defSym);
+                if (defSym.SymbolSource != Symbol.Source.Variable) {
+                    // not expected to happen
+                    Debug.WriteLine("Found local variable with bad source: " +
+                        defSym.SymbolSource);
+                    continue;
+                }
+                outLvt.AddOrReplace(defSym);
             }
             return true;
         }