[OMPIRBuilder] Fix shared clause for task construct

This patch fixes the shared clause for the task construct with multiple
shared variables. The shareds field in the kmp_task_t is not an inline
array in the struct, rather it is a pointer to an array. With an inline
array, the pointer dereference to the outlined function body of the task
would segmentation fault when accessed by the runtime.

Reviewed By: kiranchandramohan, jdoerfert

Differential Revision: https://reviews.llvm.org/D158462