Provide an explicit JS type for DateTypeSymbols

This reduces the size of the .d.ts file for javascript/closure/i18n/datetimesymbolsext.js from 568.2kb to 272.7kb by not inferring a separate type for each constant.

RELNOTES: Small change to datetimesymbols to optimize clutz output.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=276530637

Author: nreid260