[NFC] Use llvm::SmallVector to workaround XL compiler problem on AIX

LLVM is recommending to use SmallVector (that is, omitting the N), in the
absence of a well-motivated choice for the number of inlined elements N.

However, this doesn't work well with XL compiler on AIX since some header(s)
aren't properly picked up with it. We need to take a further look into the real
issue underneath and fix it in a later patch.

But currently we'd like to use this patch to unblock the build compiler issue
first.

Differential Revision: https://reviews.llvm.org/D98265