[clang] Pass FoundDecl to DeclRefExpr creator for operator overloads

Without the "found declaration" it is later not possible to know where the operator declaration
was brought into the scope calling it.

The initial motivation for this fix came from #55095. However, this also has an influence on
`clang -ast-dump` which now prints a `UsingShadow` attribute for operators only visible through
`using` statements. Also, clangd now correctly references the `using` statement instead of the
operator directly.

Reviewed By: shafik

Differential Revision: https://reviews.llvm.org/D129973