Gardening: Address -Wunused-but-set-variable warnings.

Author: tshortli

include/swift/Remote/MetadataReader.h

@@ -1481,7 +1481,7 @@ class MetadataReader {
       return 0;
     };
 
-    switch (auto kind = flags.getKind()) {
+    switch (flags.getKind()) {
     case ContextDescriptorKind::Module:
       baseSize = sizeof(TargetModuleContextDescriptor<Runtime>);
       break;

lib/Demangling/Demangler.cpp

@@ -2810,7 +2810,7 @@ NodePointer Demangler::demangleThunkOrSpecialization() {
   switch (char c = nextChar()) {
     // Thunks that are from a thunk inst. We take the TT namespace.
     case 'T': {
-      switch (char c = nextChar()) {
+      switch (nextChar()) {
       case 'I':
         return createWithChild(Node::Kind::SILThunkIdentity, popNode(isEntity));
       case 'H':

stdlib/public/runtime/LibPrespecialized.cpp

@@ -686,7 +686,7 @@ void _swift_validatePrespecializedMetadata() {
 
     auto result = swift_getTypeByMangledName(MetadataState::Complete,
                                              mangledName, nullptr, {}, {});
-    if (auto *error = result.getError()) {
+    if (result.getError()) {
       fprintf(stderr,
               "Prespecializations library validation: unable to build metadata "
               "for mangled name '%s'\n",

stdlib/public/runtime/MetadataLookup.cpp

@@ -164,7 +164,7 @@ ResolveAsSymbolicReference::operator()(SymbolicReferenceKind kind,
       break;
 
     default:
-      if (auto typeContext = dyn_cast<TypeContextDescriptor>(descriptor)) {
+      if (isa<TypeContextDescriptor>(descriptor)) {
         nodeKind = Node::Kind::TypeSymbolicReference;
         isType = true;
         break;