Implement GetComment API (dotnet/corefx#35705)
* CoreFx dotnet/corefx#33347: Implement GetComment API
* Generate References as per correct steps
* Exclude delims & line separators, new unescape test
* Revert consume comment changes, test case fixes
* Partial revert of previous incorrect online merge
* fixed another online merge conflict resolution slippage
* Intermediate rebase from master
* Reworked files
* Reworked files 2
* Fix coding sytle as per review comments
Commit migrated from https://github.com/dotnet/corefx/commit/
53aaaf3c848e4e737420de2b46f7cace397d730a