[Backport to 19] Add SPV_INTEL_16bit_atomics extension (#3424)

[vmaksimo]

This continues #3343 and reflects specification update, including extension renaming. Specification: intel/llvm#20009

[vmaksimo]

About memory scope change - it is expected, cause the fix for correct scope translation was added only in llvm-20 release branch 630a90a#diff-ba003da77a80df1db76fb378a873a80e758591d72bf82a46fc90069b2a947990R1904
And for this and earlier release branches we use Device scope by default:

SPIRV-LLVM-Translator/lib/SPIRV/SPIRVWriter.cpp

Lines 1908 to 1917 in 1e4166d

	SPIRVValue *LLVMToSPIRVBase::transAtomicStore(StoreInst *ST,
	SPIRVBasicBlock *BB) {
	SmallVector<StringRef> SSIDs;
	ST->getContext().getSyncScopeNames(SSIDs);
	spv::Scope S;
	// Fill unknown syncscope value to default Device scope.
	if (!OCLStrMemScopeMap::find(SSIDs[ST->getSyncScopeID()].str(), &S)) {
	S = ScopeDevice;
	}