When performing overload resolution during code completion,
clang will allow incomplete substitutions in more places
than would be allowed for valid code, because for completion to work well,
it needs clang to keep going so it can explore the space of possibilities.
Notably, we accept instantiating declarations will null template arguments,
and this works fine, except that when lazily loading serialzied templated
declarations, the template argument hasher assumes null arguments can't
be used.
This patch makes the hasher happily accept that.
Fixes https://github.com/llvm/llvm-project/issues/139019
7759bb57c2
