Remove LINQ usage in language service methods (#737)

* Remove excess LINQ usage in LanguageService.cs

* [Style] Simplify method bodies

* Replace LINQ aggregate call

* [Style] Change LanguageService fields to underscore style

* [Style] Convert non-obvious var usage

* [Style] Use nameof for parameter validation

* [Style] Address PR feedback from @tylerl0706

Author: rjmholt