[OpenMP][NFC] Move mapping related code into OpenMP/Mapping.cpp (#75239)

DeviceTy provides an abstraction for "middle-level" operations that can
be done with a offload device. Mapping was tied into it but is not
strictly necessary. Other languages do not track mapping, and even
OpenMP can be used completely without mapping. This simply moves the
relevant code into the OpenMP/Mapping.cpp as part of a new class
MappingInfoTy. Each device still has one, but it does not clutter the
device.cpp anymore.