Auto merge of #15609 - WaffleLapkin:undup_sema, r=Veykril

Remove most of the duplication from `Semantics{,Impl}` via deref

See https://rust-lang.zulipchat.com/#narrow/stream/185405-t-compiler.2Frust-analyzer/topic/Semantics.20vs.20SemanticsImpl/near/390795952

Author: bors