[NFC] Add size inference to to_vector

A default calculated size for SmallVector was added in
https://reviews.llvm.org/D92522 after discussion in
https://groups.google.com/g/llvm-dev/c/Z-VwNCTRGSg, but to_vector still
requires an explicit size. This patch adds the default size to to_vector
as well, so that this case doesn't unnecessarily force users to pick an
arbitrary size.

Reviewed By: silvas, dblaikie

Differential Revision: https://reviews.llvm.org/D112968