Looking up line numbers with the JITLineInfoTable would sometimes give
wrong answers. Fix these bugs and add a cctest for this data structure.

Also do some cleanup while we're here like inlining the (empty)
constructor and destructor and removing the empty() method which is
only used unnecessarily anyway, to make the contract of
GetSourceLineNumber a bit clearer.

Also rename the data structure to SourcePositionTable, because it
doesn't just provide info for JIT code, but also bytecode, and 'Info'
is pretty ambiguous.

Bug: v8:7018
Change-Id: I126581c844d85df6b2b3f80f2f5acbce01c16ba1
Reviewed-on: https://chromium-review.googlesource.com/1006795Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52571}