[Sema] Add SyntacticElementTarget::getPattern

And use it for walking the SyntacticElementTarget
such that patterns for uninitialized vars are
walked too.

Author: hamishknight

include/swift/Sema/SyntacticElementTarget.h

@@ -591,6 +591,19 @@ class SyntacticElementTarget {
     expression.expression = expr;
   }
 
+  Pattern *getPattern() const {
+    if (auto *pattern = getAsUninitializedVar())
+      return pattern;
+
+    if (isForInitialization())
+      return getInitializationPattern();
+
+    if (kind == Kind::forEachPreamble)
+      return forEachStmt.pattern;
+
+    return nullptr;
+  }
+
   void setPattern(Pattern *pattern) {
     if (kind == Kind::uninitializedVar) {
       assert(uninitializedVar.declaration.is<Pattern *>());

lib/Sema/SyntacticElementTarget.cpp

@@ -337,8 +337,8 @@ SyntacticElementTarget::walk(ASTWalker &walker) const {
   SyntacticElementTarget result = *this;
   switch (kind) {
   case Kind::expression: {
-    if (isForInitialization()) {
-      if (auto *newPattern = getInitializationPattern()->walk(walker)) {
+    if (auto *pattern = getPattern()) {
+      if (auto *newPattern = pattern->walk(walker)) {
         result.setPattern(newPattern);
       } else {
         return std::nullopt;