[Windows] Replace TrapUnreachable with an int3 insertion pass

This is an alternative to D66980, which was reverted. Instead of
inserting a pseudo instruction that optionally expands to nothing, add a
pass that inserts int3 when appropriate after basic block layout.

Reviewers: hans

Differential Revision: https://reviews.llvm.org/D67201

llvm-svn: 371466