This CL changes the generated C++ code for LabeledStatementBlocks to
only emit labels if they are used.

Prior to this CL, when a label was only used on one path of an
if constexpr expression, and not at all anywhere else,
the try/label construct would BIND a label that was not used,
causing a CSA verification error.

R=tebbi@chromium.org

Change-Id: Ia81a0cd081b84528c95bbdbdb98b9ab51928e13f
Reviewed-on: https://chromium-review.googlesource.com/1057247Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Daniel Clifford <danno@chromium.org>
Commit-Queue: Simon Zünd <szuend@google.com>
Cr-Commit-Position: refs/heads/master@{#53173}