This reduces the memory consumption of SourcePositionTable by ca. 2/3.
Over Octane, this reduces the source position table memory consumption
from ~370kB to ~115kB, which makes it ca. 10% of the total bytecode size
(~1.1MB)

----------------

Reland CL in order to relive the glory days, and also fix memory leak w/ ENABLE_SLOW_CHECKS.

SourcePositionTableBuilder used to have a no destructor since everything
was zone allocated. But if ENABLE_SLOW_CHECKS, it has a heap allocated member
and thus needs a proper constructor. ASAN thankfully notices this, and V8 no
longer builds since this is called during mksnapshot.

Breakge example: http://build.chromium.org/p/client.v8/builders/V8%20Linux64%20ASAN%20arm64%20-%20debug%20builder/builds/4829

R=jochen@chromium.org, yangguo@chromium.org, rmcilroy@chromium.org
BUG=v8:4690
LOG=y

Committed: https://crrev.com/a6f41f7b8226555c5900440f6e3092b3545ee0f6
Cr-Commit-Position: refs/heads/master@{#34250}

patch from issue 1704943002 at patchset 200001 (http://crrev.com/1704943002#ps200001)

Review URL: https://codereview.chromium.org/1731883003

Cr-Commit-Position: refs/heads/master@{#34256}