Improve context constructor to throw exception if devices don't belong to the same platform

[AlexeySachkov]

llvm/sycl/source/context.cpp

Line 43 in 07a3616

// TODO also check that devices belongs to the same platform

Description of the corresponding constructor from the SYCL 1.2.1 spec:

Constructs a SYCL context instance using the SYCL device(s) in the deviceList parameter as the associated SYCL device(s) and the SYCL platform associated with each SYCL device in the deviceList parameter as the associated SYCL platform. This requires that all SYCL devices in the deviceList parameter have the same associated SYCL platform. The constructed SYCL context will use the asyncHandler parameter to handle exceptions.

[AlexeySachkov]

Fixed by #1092