[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