IRMover: Steal arguments when moving functions, NFC

Instead of copying arguments from the source function to the
destination, steal them.  This has a few advantages.

  - The ValueMap doesn't need to be seeded with (or cleared of)
    Arguments.

  - Often the destination function won't have created any arguments yet,
    so this avoids malloc traffic.

  - Argument names don't need to be copied.

Because argument lists are lazy, this required a new
Function::stealArgumentListFrom helper.

llvm-svn: 265519