[dllexport] odr-use constexpr default args for constructor closures

InstantiateDefaultCtorDefaultArgs() is supposed to mark default
constructor args as odr-used, since those args will be used when
emitting the constructor closure.

However, constexpr vars were not getting odr-used since
DoMarkVarDeclReferenced() defers them in MaybeODRUseExprs, and the code
was calling CleanupVarDeclMarking() which discarded those uses instead
of processing them.

(This came up in Chromium, crbug.com/1312086)

Differential revision: https://reviews.llvm.org/D123405