[clang] [clang] Be const-correct with all uses of `Module *`. (PR #93493)

[Erich Keane via cfe-commits]

erichkeane wrote:

> One suggestion, if you are going to go about this problem systematically, try to tackle the hardest problems first.
> 
> For example: Try to make `MultiLevelTemplateArgumentList` const correct. I would like other suggestions as well.

Right, yeah.  Unfortunately const-correctness in Clang is really bad, and not really something we can fix well piecemeal without a lot of churn/issues.  Our current approach is pretty pervasive to breaking const-correctness, so any iterative/piecemeal appraoch ends up having to const-cast all over the place.  I am pretty sure a few dozen folks have attempted over the last few years, each ended up more annoyed than the last :) 

That said, this patch Is something I think is a good direction.  However, I fear(it seems?) it doesn't use any level of holistic approach to determining which functions need to mutate other than "they don't mutate today".  Which is, IMO, a worse level of const-correctness to be in.  We need to identify which functions/uses are conceptually modifying (see the `getSomething` examples above), and which aren't, then do the addition of const from there.

https://github.com/llvm/llvm-project/pull/93493