[COFF] Avoid copying of chunk vectors. NFC.

When declaring the pair variable as "auto Pair : Map", it is
effectively declared as
std::pair<std::pair<StringRef, uint32_t>, std::vector<Chunk *>>.
This effectively does a full, shallow copy of the Chunk vector,
just to be thrown away after each iteration.

Differential Revision: https://reviews.llvm.org/D52051

llvm-svn: 342205

lld/COFF/Writer.cpp

@@ -469,7 +469,7 @@ void Writer::createSections() {
   // '$' and all following characters in input section names are
   // discarded when determining output section. So, .text$foo
   // contributes to .text, for example. See PE/COFF spec 3.2.
-  for (auto Pair : Map) {
+  for (auto &Pair : Map) {
     StringRef Name = getOutputSectionName(Pair.first.first);
     uint32_t OutChars = Pair.first.second;