The Platform base class now maintains a list of trap handlers
aka asynchronous signal handlers, which subclasses should fill
in as appropriate.  For most Unix user process environments,
the one entry in this list is _sigtramp.  For bare-board and
kernel environments, there will be different sets of trap 
handlers.

The unwinder needs to know when a frame is a trap handler 
because the rules it enforces for the frame "above" the
trap handler is different from most middle-of-the-stack frames.

<rdar://problem/15835846> 


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@201300 91177308-0d34-0410-b5e6-96231b3b80d8