[mlir][LLVM] Add passthrough import support

[Dinistro]

Summary

This commit extends the LLVMIR import to support function attributes by
adding them to the passthrough array attribute.

Depends on D142013

[gysit]

Nice, LGTM modulo comments!

mlir/lib/Target/LLVMIR/ModuleImport.cpp

1431

nit: a->attr or funcAttr?

1432

nit: hande -> handle maybe say "Skip the memory attribute since the funcOp has an explicit memory attribute."

1466

let's use llvm_unreachable here?

mlir/test/Target/LLVMIR/Import/basic.ll

10–11

nit: time to remove the FIXME :)?

[Dinistro]

address review comments

[Dinistro]

change unreachable message