Translate AST to Hydrogen missing position

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}

src/hydrogen.h

@@ -1889,6 +1889,9 @@ class HGraphBuilder {
   // the SourcePosition assuming that this position corresponds to the
   // same function as current position_.
   SourcePosition ScriptPositionToSourcePosition(int position) {
+    if (position == RelocInfo::kNoPosition) {
+      return SourcePosition::Unknown();
+    }
     SourcePosition pos = position_;
     pos.set_position(position - start_position_);
     return pos;