This patch adds more flexibility to the original VFABI demangling API.
1. Allow demangling an empty parameter list, which is explicitly allowed
by x86 VFABI doc. See section 2.6 of https://sourceware.org/glibc/wiki/libmvec?action=AttachFile&do=view&target=VectorABI.txt
2. Allow passing the module as a nullptr, so that we don't check the
existence of the demangled vector function name. This is useful when
we want to create the widened function according to the demangling
Signed-off-by: Yilong Guo <email@example.com>