rustc_trans: Fix type projection debuginfo

[erickt]

Closes #20797

[rust-highfive]

r? @pnkfelix

(rust_highfive has picked a reviewer for you, use r? to override)

[erickt]

@sfackler: should be fixed now.

@michaelwoerister: you should probably check this out too since I'm new to this area of the code.

[michaelwoerister]

Looks good, thanks Erick!
I'm fine with this being merged to get rid of the ICE. But I'm not sure it's entirely correct to do it that way. I'll think about it make another PR if necessary.

[kinghajj]

I'd like to note that while this does fix erickt's example from #20797, my original one still causes an ICE when compiling with this branch merged into the master.