Spirv-Link fails to combine modules with OpenCL extern kernel declarations

The linker complains that the entrypoint is already defined. Perhaps this should simply be allowed for kernels.

Start by reproducing the SPIR-V linker failure with modules containing OpenCL extern kernel declarations, then trace how the linker detects duplicate entrypoints. The issue names no files or tests, so first locate the relevant linker code and existing diagnostics. Done means the valid modules combine without the duplicate-entrypoint error and the behavior is covered by a regression test.

Written by the indexing model from the issue text.