swift234

Try to ensure we generate unique trap locations.

In 2e3c0b6, code was added to emit unique trap blocks for each
cond_fail, in order to make post-mortem debugging simpler (e.g. stack
traces have correct line/column information for the trapping location,
and it's easy to trace back to the specific jump that reaches the trap).

We didn't, however, do anything to ensure that LLVM wouldn't merge these
back together again. This is an attempt to do exactly that, after seeing
BranchFolding in the code generator merging traps into a single block.

The idea here is to emit empty inline asm strings marked as
side-effecting, and taking a unique integer argument. These come before
the trap call, so they should block any valid attempt at merging the
blocks back together.

Ideally trap would take an argument which uniquely identifies it, but
that isn't possible today.

This solution is potentially brittle in that in theory LLVM could still
merge the trap/unreachable and then branch to those after the unique asm
calls. We cannot fix that by putting another asm call after the trap,
because LLVM's CFG simplification will delete code after a trap.

rdar://problem/25216969