spirv-val feature request: validate full pipeline cross-stage linkage

Feature request: I frequently find myself in a situation where each of the individual SPIR-V files in my pipeline pass spirv-val, but validation layers will find cross-stage linkage errors (i.e. mismatch between input/output of stages). It would be super useful if there was way I could pass in multiple SPIR-V files (VS/FS/TCS/TES/GS) and have spirv-val check for cross-stage errors as happens in the validation layers so these kinds of errors could be caught offline.

Start by tracing how spirv-val accepts and validates individual SPIR-V files, then compare that path with the cross-stage checks performed by the validation layers. Define the supported stage combinations and linkage mismatches to report; done means multiple pipeline files can be validated offline with cross-stage errors detected.

Written by the indexing model from the issue text.