- In debug builds, 'assert(<expr>)' evaluates and aborts execution
  if the provided Torque expression is false at runtime.
  assert(<expr>) supports the same set of expressions protocols
  as Toruqe's if statement, i.e. both bool values and BranchIf-
  style tests. Upon failure, the assertion prints the Torque
  source code of the failed expression, not the generated CSA
  code.
- 'unreachable' calls CSA's Unreachable() and signals to Torque
  that code execution cannot continue (i.e. its statement
  returns the 'never' type). In debug builds, the line number
  and position of the statement are printed before breaking.
- 'debug' calls CSA's DebugBreak(). In debug builds, the line
  number and position of the 'debug' are printed before breaking.

Change-Id: I4efd052536bb402c097a0d5f7be56e154b5b3676
Reviewed-on: https://chromium-review.googlesource.com/1042570
Commit-Queue: Daniel Clifford <danno@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52984}