[Support] Make readNext default to unaligned (#88808)

Without this patch, you would typically use readNext as:

  readNext<uint32_t, llvm::endianness::little, unaligned>(Ptr)

which is quite mouthful.  Since most serialization/deserialization
operations are unaligned accesses, this patch makes the alignment
template parameter default to unaligned, allowing us to say:

  readNext<uint32_t, llvm::endianness::little>(Ptr)

I'm including a few examples of migration in this patch.  I'll do the
rest in a separate patch.

Note that writeNext already has the same trick for the alignment
template parameter.