[Libomptarget] Replace std::vector with llvm::SmallVector

The runtime makes some use of `std::vector` data structures. We should
be able to replace these trivially with `llvm::SmallVector` instead.
This should allow us to avoid heap allocations in the majority of cases
now.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D130927

GitOrigin-RevId: 51bda3a0e7ac0285cf116b5cfd1393a3340974b5
5 files changed