[OpenMP][Plugin] Minor adjustments to ResourcePool

This patch makes some minor adjustments to `ResourcePool`:
- Don't initialize the resources if `Size` is 0 which can avoid assertion.
- Add a new interface function `clear` to release all hold resources.
- If initial size is 0, resize to 1 when the first request is encountered.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D116340