[Walker] Don't visit VarDecls encountered in freestanding macro expansions

VarDecls are always walked via their pattern bindings, so we end up
with double-visitation if we also visit them here.

Fixes rdar://109376102.

Author: DougGregor

lib/AST/ASTWalker.cpp

@@ -460,7 +460,8 @@ class Traversal : public ASTVisitor<Traversal, Expr*, Stmt*,
     if (shouldWalkExpansion) {
       MED->visitAuxiliaryDecls([&](Decl *decl) {
         if (alreadyFailed) return;
-        alreadyFailed = inherited::visit(decl);
+        if (!isa<VarDecl>(decl))
+          alreadyFailed = inherited::visit(decl);
       });
       MED->forEachExpandedExprOrStmt([&](ASTNode expandedNode) {
         if (alreadyFailed) return;

test/Macros/macro_expand.swift

@@ -52,6 +52,9 @@ macro freestandingWithClosure<T>(_ value: T, body: (T) -> T) = #externalMacro(mo
 @freestanding(expression) macro stringify<T>(_ value: T) -> (T, String) = #externalMacro(module: "MacroDefinition", type: "StringifyMacro")
 
 @freestanding(declaration, names: arbitrary) macro bitwidthNumberedStructs(_ baseName: String, blah: Bool) = #externalMacro(module: "MacroDefinition", type: "DefineBitwidthNumberedStructsMacro")
+
+@freestanding(declaration, names: named(value)) macro varValue() = #externalMacro(module: "MacroDefinition", type: "VarValueMacro")
+
 #endif
 
 #if TEST_DIAGNOSTICS
@@ -409,3 +412,8 @@ func testFreestandingClosureNesting() {
     #coerceToInt(2)
   })
 }
+
+// Freestanding declaration macros that produce local variables
+func testLocalVarsFromDeclarationMacros() {
+  #varValue
+}