Fix non determinism in code generation.

GenerateJavaScriptAst.FixNameClashesVisitor fixes name clashes in
a non deterministic order due to iteration over a HashMultimap.

This fix makes the iteration order deterministic by using a
LinkHashMultimap instead.

Change-Id: Iaa878666f4fbae7cf235d3d53f223d8a08af6e4f
(cherry picked from commit d94b402596302381061b72698bc706257307a4e7)