Avoid some copies by using const references.

clang-tidy's performance-unnecessary-copy-initialization with some manual
fixes. No functional changes intended.

llvm-svn: 270988
diff --git a/llvm/tools/llvm-dwarfdump/llvm-dwarfdump.cpp b/llvm/tools/llvm-dwarfdump/llvm-dwarfdump.cpp
index 415de6c..fb6c7f4 100644
--- a/llvm/tools/llvm-dwarfdump/llvm-dwarfdump.cpp
+++ b/llvm/tools/llvm-dwarfdump/llvm-dwarfdump.cpp
@@ -158,7 +158,7 @@
 
   // Expand any .dSYM bundles to the individual object files contained therein.
   std::vector<std::string> Objects;
-  for (auto F : InputFilenames) {
+  for (const auto &F : InputFilenames) {
     auto Objs = expandBundle(F);
     Objects.insert(Objects.end(), Objs.begin(), Objs.end());
   }