This patch translates RelocInfo::kNoPosition to SourcePosition::Unknown()
in constructing the Hydrogen graph from the parser's output. The translation
is done to increase the flexibility of the parser to desugar more things
that don't have natural locations.

R=bmeurer@chromium.org
BUG=v8:4377
LOG=N

Review URL: https://codereview.chromium.org/1313443002

Cr-Commit-Position: refs/heads/master@{#30337}