[OpenMP] Require trivially copyable type for mapping

A trivially copyable type provides a trivial copy constructor and a trivial
copy assignment operator. This is enough for the runtime to memcpy the data
to the device. Additionally there must be no virtual functions or virtual
base classes and the destructor is guaranteed to be trivial, ie performs
no action.
The runtime does not require trivial default constructors because on alloc
the memory is undefined. Thus, weaken the warning to be only issued if the
mapped type is not trivially copyable.

Differential Revision: https://reviews.llvm.org/D71134