A little more work on the compact unwind dumper.

UNWIND_X86_64_MODE_STACK_IND mode is almost correct; extra stack
space allocated before the reg saves isn't handled right.  Still a
little wobbily on the file addresses of functions.  Finally understand
how the 6 registers that may be saved are ordered in just 10 its
of space -- the Lehmer code for the registers is derived and then
the sequence is encoded in a variable base number.  Added some
comments with references to what the code is doing so it'll be
easier for others to track down.

llvm-svn: 222884