[PCH/Modules] The iterator may become invalidated because a new macro can be added
while deserializing a macro, make sure to copy/move what we need from it.

Fixes clang-x86_64-debian-fast bot.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172629 91177308-0d34-0410-b5e6-96231b3b80d8