Attach attribute "trap-func-name" to call sites of llvm.trap and llvm.debugtrap.

This is needed to use clang's command line option "-ftrap-function" for LTO and
enable changing the trap function name on a per-call-site basis.

rdar://problem/21225723

Differential Revision: http://reviews.llvm.org/D10831

llvm-svn: 241306