proposal: add sem package to perform semantic analysis on LLVM IR modules

[mewmew]

The addition of a semantic analysis package has been discussed at different places in the past. This issue tracks the implementation and design considerations of such a package.

Wish list for package llir/llvm/sem.

1. type checking of an LLVM IR module (ir: proposal: typecheck more things #65).
2. identify duplicate names of global identifiers (ir: rename duplicate named functions(only when funciton in module) #76).
3. ...
4. profit

@dannypsnl and @pwaller, feel free to propose additions to this list. Then we can figure out how to design and implement such a package.

[mewmew]

As noted by @pwaller in #65 (comment):

We do already have access to a functioning verifier (opt -verify).

As such, we could/should probably rely on the already implemented LLVM IR verification of opt rather than re-implementing this in Go, since our verification would quite likely fall out of sync with the official implementation. Closing this issue for now. If there are significant reasons why we'd still want to do this in Go, rather than just invoking the opt tool, we can re-open this issue. Closing for now.