[clang] Fix consteval operators in template contexts

Clang used to reject consteval operators if they're used inside a
template due to TreeTransform putting two different `DeclRefExpr`
expressions for the same reference of the same operator declaration into
`ReferenceToConsteval` set.
It seems there was an attempt to not rebuild the whole operator that
never succeeded, so this patch just removes this attempt and
problemating referencing of a `DeclRefExpr` that always ended up
discarded.

Fixes https://github.com/llvm/llvm-project/issues/62886

Reviewed By: cor3ntin

Differential Revision: https://reviews.llvm.org/D151553