keep a shared pointer to their disassembler.  This
is important for the LLVM-C disassembler because
it needs to lock its parent in order to disassemble
itself.

This means that every interface that returned a
Disassembler* needs to return a DisassemblerSP, so
that the instructions and any external owners share
the same reference count on the object.  I changed
all clients to use this shared pointer, which also
plugged a few leaks.

<rdar://problem/12002822>

llvm-svn: 161123