[OpenMP] Add function for setting LIBOMPTARGET_INFO at runtime

Summary:
This patch adds a new runtime function __tgt_set_info_flag that allows the
user to set the information level at runtime without using the environment
variable. Using this will require an extern function, but will eventually be
added into an auxilliary library for OpenMP support functions.

This patch required moving the current InfoLevel to a global variable which must
be instantiated by each plugin.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D100774