build abstract consts without typeck for fully qualified associated consts

[BoxyUwU]

When we build an abstract const for the const arg in foo::<{ <T as Trait>::ASSOC }>() we currently typeck the anon constant and then build an abstract const by walking thir. this PR adds a special case for when the anon const is a fully qualified assoc const, instead of typeck'ing and using thir we will just lower it to to defid + substs pair where the defid is the associated item on the trait (the same result using thir would give us but without the cycles :P).

r? @lcnr

[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

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configure: rust.channel         := nightly
configure: rust.debug-assertions := True
configure: llvm.assertions      := True
configure: dist.missing-tools   := True
configure: build.configure-args := ['--enable-sccache', '--disable-manage-submodu ...
configure: writing `config.toml` in current directory
configure: 
configure: run `python /checkout/x.py --help`
configure: 
---
* highest error code: E0785
Found 501 error codes
Found 0 error codes with no tests
Done!
tidy error: /checkout/compiler/rustc_typeck/src/collect.rs:94: TODO is deprecated; use FIXME
some tidy checks failed



command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/rust-tidy" "/checkout" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "/checkout/obj/build" "16"


Build completed unsuccessfully in 0:00:12

[bors]

☔ The latest upstream changes (presumably #90119) made this pull request unmergeable. Please resolve the merge conflicts.

[BoxyUwU]

gonna close this I dont think theres any point keeping it open, my point has been proven 🤣