[Coverage][NFC] Avoid visiting non-unique `OpaqueValueExpr` (#88910)

Only unique `OpaqueValueExpr`s should be handled in the mapping builder,
as
[discussed](https://github.com/llvm/llvm-project/pull/85837#discussion_r1542056451)
in #85837. However, `getCond()` returns non-unique `OpaqueValueExpr` for
`BinaryConditionalOperator` (because it is also used as the "true"
branch expression). Use `getCommon()` instead so as to bypass the
`OpaqueValueExpr`.