[llvm-jitlink] Use SmallVectorImpl when referencing StubInfos (NFC)

The element type is declared as SmallVector<T, 1>, but we assign to
SmallVector<T> &. These types are not the same on 32-bit systems,
resulting in a compilation error.

Fix this by using SmallVectorImpl<T> & instead, which is independent
of the small size.