Add a new target setting, trap-handler-names, where a user can

Add a new target setting, trap-handler-names, where a user can
specify a list of functions which should be treated as trap handlers.
This will be primarily useful to people working in non-user-level
process debugging - kernels and other standalone environments.
For most people, the trap handler functions provided by the Platform
plugin will be sufficient.

<rdar://problem/15835846>, <rdar://problem/15982682>

llvm-svn: 201386