Serialization: use range based for loop (NFC)

Just a small clean up noticed when doing post-commit review of Duncan's
previous change for ModuleFile memory ownership semantics.  NFC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@293556 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed