[LLVM][ADT] Explicitly convert size_t values to SmallVector's size type (#77939)

Multiple places rely on implicit conversion when assigning 'size_t'
values to the member fields (size or capacity) of SmallVector.

Depending on the platform / compiler configuration, this may result in
narrowing conversion warnings (especially given that the size type of
SmallVector's member fields is determined based on type T - in
SmallVector<T>). To avoid the problem altogether, make the conversions
explicit.

Co-authored-by: Orest Chura <orest.chura@intel.com>