Make llvm.eh.begincatch use an outparam

Ultimately, __CxxFrameHandler3 needs us to put a stack offset in a
table, and it will take responsibility for copying the exception object
into that slot. Modelling the exception object as an SSA value returned
by begincatch isn't going to work in general, so make it use an output
parameter.

Reviewers: andrew.w.kaylor

Differential Revision: http://reviews.llvm.org/D7920

llvm-svn: 231086