Add a verifier check that rejects non-distinct DISubprogram function

attachments. They would crash the backend, which expects all
DISubprograms that are not part of the type system to have a unit field.

Clang right before https://reviews.llvm.org/D79967 would generate this
kind of broken IR.

rdar://problem/69534688

Thanks to Fangrui for fixing an assembler test I had missed!

https://reviews.llvm.org/D88270