Removed lookup of TemplateStringsArray for non-ES6 targets.

This will enable custom ES3/ES5 lib.d.ts files that omit the TemplateStringsArray type, but don't need it anyway.
Baselines, using 'unknown' as a default type.
Fixed issue in harness where lib.d.ts errors are note counted.
Added tests for defining TemplateStringsArray in certain contexts.
Merge pull request #1160 from Microsoft/weDontNeedNoTemplateStringsArrayPreEs6

Default to 'unknown' for the template strings array type in pre-ES6 targets.