This CL is a proposal to add "checked" casts (CAST in CSA) to the Torque language.
The CL adds the "unsafe_cast<>" operator that emits a "CAST".

Example:

let n: Number = ...;
...
if (TaggedIsSmi(n)) {
  let m: Smi = unsafe_cast<Smi>(n);
  ...
}

The cast wont incur a runtime overhead now.

R=tebbi@chromium.org

Change-Id: I9fca90d1d11e61617ba0270e5022fd66200e2195
Reviewed-on: https://chromium-review.googlesource.com/1070151
Commit-Queue: Simon Zünd <szuend@google.com>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53416}