[libc++abi][NFC] Move PODSmallVector definition to the top of ItaniumDemangle.h

This change is needed to clean the non-relevant parts of diff
from https://reviews.llvm.org/D107712

Differential Revision: https://reviews.llvm.org/D107771

GitOrigin-RevId: db7c68d80807bcb5127e9acc3818a3ae9855108c
1 file changed