[vm] Return token position from BuildStatement.

Not every statement has a serialized position we can use for coverage.
For example, the otherwise part of an if statement doesn't have a
position. So for branch coverage, BuildStatement needs to return a token
position of the statement, so that we can pass it to the record coverage
instruction.

For example, if statement's otherwise fragment will look like this:

TokenPosition other_pos = TokenPosition::kNoSource;
Fragment otherwise_statement = BuildStatement(&other_pos);
otherwise_fragment += flow_graph_builder_->RecordCoverage(other_pos);
otherwise_fragment += otherwise_statement;

TEST=CI
Change-Id: Ibfb7b81bf0b98b12e850e60f565fcc0ced521f04
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/223900
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Liam Appelbe <liama@google.com>