OpenMP: Avoid using SmallVector::set_size()

Update `OpenMPIRBuilder::collapseLoops()` to call `resize()` instead of
`set_size()`. The latter asserts on capacity limits and cannot grow,
which seems likely to be unintentional here (if it is, I think a local
assertion would be good for clarity).

Also update `CodeGenFunction::EmitOMPCollapsedCanonicalLoopNest()` to
use `pop_back_n()` instead of `set_size()`.

Differential Revision: https://reviews.llvm.org/D115378