[OpenMP] Encapsulate more in checkDeviceAndCtors

This patch just encapsulates some repeated code.  To do so, it
relocates some functions from interface.cpp to omptarget.cpp.  It also
adjusts them to the LLVM coding style.

This patch is almost NFC except some `DP` messages are a bit
different.  For example, messages like "Entering target region" are
now emitted even if offload is disabled, but a subsequent "Offload is
disabled" is then emitted.

Reviewed By: jdoerfert, grokos

Differential Revision: https://reviews.llvm.org/D97908