Patch by Vadim Chugunov

Win64 stack unwinder gets confused when execution flow "falls through" after
a call to 'noreturn' function. This fixes the "missing epilogue" problem by 
emitting a trap instruction for IR 'unreachable' on x86_x64-pc-windows.

A secondary use for it would be for anyone wanting to make double-sure that
'noreturn' functions, indeed, do not return.

llvm-svn: 206684