This CL improves reported source range precision in a couple of ways:

Source ranges are now standardized to consist of an inclusive start
index and an exclusive end index (similar to what's reported for
functions). For example:

0123456789  // Offset.
{ f(); }    // Block represented as range {0,8}.

Duplicate singleton ranges (i.e. same start and end offsets) are now
merged (this only becomes relevant once jump statement coverage is
added). For example:

for (.) break;  // Break- and loop continuation have same positions.

SourceRangeScope incorrectly collected starting position
(unconditionally) and end position (when no semi-colon was present).

01234567890123  // Offset.
for (.) break   // Loop body range is {8,13}, was {6,9}.

Bug: v8:6000
Change-Id: I62e7c70cc894a20f318330a2fbbcedc47da2b5db
Reviewed-on: https://chromium-review.googlesource.com/541358
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46095}