Fix GCC warnings. NFC.

Author: darkbuck

clang/lib/AST/ItaniumMangle.cpp

@@ -641,7 +641,7 @@ void CXXNameMangler::mangle(GlobalDecl GD) {
   //            ::= <data name>
   //            ::= <special-name>
   Out << "_Z";
-  if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(GD.getDecl()))
+  if (isa<FunctionDecl>(GD.getDecl()))
     mangleFunctionEncoding(GD);
   else if (const VarDecl *VD = dyn_cast<VarDecl>(GD.getDecl()))
     mangleName(VD);

clang/lib/Index/USRGeneration.cpp

@@ -388,7 +388,7 @@ static const ObjCCategoryDecl *getCategoryContext(const NamedDecl *D) {
   if (auto *ICD = dyn_cast<ObjCCategoryImplDecl>(D->getDeclContext()))
     return ICD->getCategoryDecl();
   return nullptr;
-};
+}
 
 void USRGenerator::VisitObjCMethodDecl(const ObjCMethodDecl *D) {
   const DeclContext *container = D->getDeclContext();