[Flang][Lower] NFC: Replace SmallVector with more suitable alternatives (#85227)

In this patch some uses of `llvm::SmallVector` in Flang's lowering to
MLIR are replaced by other types (i.e. `llvm::ArrayRef` and
`llvm::SmallVectorImpl`) which are intended for these uses. This
generally prevents relying on always passing small vectors with a
particular number of elements in the stack.