Fix a memory leak in InstructionLLVMC where it held onto a strong reference to...

Fix a memory leak in InstructionLLVMC where it held onto a strong reference to the DisassemblerLLVMC which in turn had a vector of InstructionSP causing the strong cycle. This is fixed now.

Rules are as follows for internal code using lldb::DisassemblerSP and lldb::InstructionSP:
1 - The disassembler needs to stay around as long as instructions do as the Instruction subclass now has a weak pointer to the disassembler
2 - The public API has been fixed so that if you get a SBInstruction, it will hold onto a strong reference to the disassembler in a new InstructionImpl class

This will keep code like like: 

inst = lldb.target.ReadInstructions(frame.GetPCAddress(), 1).GetInstructionAtIndex(0)
inst.GetMnemonic()

Working as expected (not the SBInstructionList() that was returned by SBTarget.ReadInstructions() is gone, but "inst" has a strong reference inside of it to the disassembler and the instruction.
                                                     
All code inside the LLDB shared library was verified to correctly hold onto the disassembler instance in all places.

<rdar://problem/24585496>

llvm-svn: 272069