[clang][CodeGen] Avoid copy of loop variable (NFC)
/llvm-project/clang/lib/CodeGen/CodeGenPGO.cpp:1140:19: error: loop variable '[_, V]' creates a copy from type 'const value_type' (aka 'const llvm::detail::DenseMapPair<const clang::Stmt *, clang::CodeGen::CounterPair>') [-Werror,-Wrange-loop-construct]
for (const auto [_, V] : *RegionCounterMap) {
^
/llvm-project/clang/lib/CodeGen/CodeGenPGO.cpp:1140:8: note: use reference type 'const value_type &' (aka 'const llvm::detail::DenseMapPair<const clang::Stmt *, clang::CodeGen::CounterPair> &') to prevent copying
for (const auto [_, V] : *RegionCounterMap) {
^~~~~~~~~~~~~~~~~~~
&
1 error generated.
