Improve code coverage

[adrianoc]

Investigate untested scenarios by looking into coverage data¹.

For instance, according to coverage data, ISymbolExtensions.ToElementKind() is missing tests around enums, interfaces, arrays, etc.

The goal of this task is:

1. Find out whether there's a way to configure codecov to ignore specific type/methods in C# (most likely ExcludeFromCodeCoverageAttribute serves this purpose)
2. Investigate common types with coverage <= 80%
3. File issues to add test for areas (type/methods) missing important coverage

Footnotes

1. The report for vnext branch (as of the date this issue has been filled) can be found here ↩