[clang] fix missing initialization of original number of expansions

When expanding undeclared function parameters, we should initialize
the original number of expansions, if known, before trying to expand
them, otherwise a length mismatch with an outer pack might not be
diagnosed.

Fixes PR56094.

Signed-off-by: Matheus Izvekov <mizvekov@gmail.com>

Differential Revision: https://reviews.llvm.org/D131802