[SYCL] Move module splitting functionality from sycl-post-link to SYCLLowerIR

[maksimsab]

This is a part of migration to New Offloading model and clang-linker-wrapper tool.
The signature of the verifyNoCrossModuleDeviceGlobalUsage is changed so that it returns an Error instead of aborting the executable.

[asudarsa]

Good refactoring Maksim.

Are there any related tests that need to be moved as well?
Also, can you please state the overall strategy here? Will ModuleSplitting be refactored into an LLVM pass? Also, please add NFC label if needed. May be change in signature makes it non NFC?

Thanks

[maksimsab]

@asudarsa
The strategy is to (1) make splitting as a library, (2) use this library in clang-linker-wrapper.
No tests are required here because no new functionality was added. Tests are expected in the second patch that adds use of the library in clang-linker-wrapper.
There is no NFC tag used because I changed the behaviour of error handling (moved abort from Module Splitter to sycl-post-link)
Here in CHECK_AND_EXIT macro:

llvm/llvm/tools/sycl-post-link/sycl-post-link.cpp

Lines 1012 to 1015 in b34ffd5

	if (DeviceGlobals) {
	auto E = Splitter->verifyNoCrossModuleDeviceGlobalUsage();
	CHECK_AND_EXIT(E);
	}

[maksimsab]

@asudarsa
I pushed the commit that makes the aborting error message the same as it was initial.

[maksimsab]

@intel/dpcpp-esimd-reviewers friendly ping.

[sarnex]

esimd owned changes lgtm, sorry for late review

[asudarsa]

A suggestion.
Going forward it might help if you can split such submissions into multiple commits: (1) Code motion without changes to error handling and (2) Handle changes to error handling. it might make it easy to triage errors later on. Thanks

[asudarsa]

LGTM. Thanks for the explanation.