refactor(mgca): Change DefKind::Const and DefKind::AssocConst to have a is_type_const flag

[JayanAXHF]

Addresses #152940

• Changed DefKind::Const and DefKind::AssocConst to have a is_type_const flag.
• changed is_type_const query to check for this flag
• removed is_rhs_type_const query

r? @BoxyUwU

[rustbot]

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

changes to the core type system

cc @lcnr

Some changes occurred in match checking

cc @Nadrieril

HIR ty lowering was modified

cc @fmease

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a, @makai410

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[JayanAXHF]

@BoxyUwU I think this is mostly complete now