In the new EH model, setup the function context and the call site info.

The DWARF exception pass uses the call site information, which is set up here. A
pre-RA pass is too late for it to use this information. So create and setup the
function context here, and then insert the call site values here (and map the
call sites for the DWARF EH pass). This is simpler than the original pass, and
doesn't make the CFG lose its SSA-ness.

It's a win-win-win-win-lose-win-win situation.

llvm-svn: 140675