Fix build breakerage on GCC 5.4:

/home/buildslave/slave_as-bldslv8/lld-perf-testsuite/llvm/tools/lld/COFF/PDB.cpp:365:51: error: 'auto' not allowed in lambda parameter
  auto DbgIt = find_if(File->getDebugChunks(), [](auto &C) {
                                                  ^~~~

http://lab.llvm.org:8011/builders/lld-perf-testsuite/builds/8717/steps/build-bin%2Flld/logs/stdio

llvm-svn: 346160