This PR adds support for the SPV_KHR_linkonce_odr extension and modifies
existing negative test with a positive check for the extension and
proper linkage type in case when the extension is enabled.
SPV_KHR_linkonce_odr adds a "LinkOnceODR" linkage type, allowing proper
translation of, for example, C++ templates classes merging during
linking from different modules and supporting any other cases when a
global variable/function must be merged with equivalent global
variable(s)/function(s) from other modules during the linking process.
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