dotnet/corefxdotnet/corefx#34200 Bring `XmlRawTextWriterGenerator.cxx` up to date...

dotnet/corefxdotnet/corefx#34200 Bring `XmlRawTextWriterGenerator.cxx` up to date. (dotnet/corefx#34224)

* dotnet/corefxdotnet/corefx#34200 Bring `XmlRawTextWriterGenerator.cxx` up to date.

* dotnet/corefxdotnet/corefx#34200 Convert `XmlRawTextWriterGenerator.cxx` to t4.

* dotnet/corefxdotnet/corefx#34200 Normalize the inconsistent use of `this`

* dotnet/corefxdotnet/corefx#34200 Make XmlRawTextWriterGenerator run at compile time.

* dotnet/corefxdotnet/corefx#34200 Generate Xml*RawTextWriterAsync.cs files.

These files appear to of never been from a generation process but are
near identical.

* dotnet/corefxdotnet/corefx#34200 Normalize `this` in XmlRawTextWriterGeneratorAsync

* dotnet/corefxdotnet/corefx#34200 Convert Html*RawTextWriter.cs to t4 template.

* dotnet/corefxdotnet/corefx#34200 Convert Text*RawTextWriter.cs to t4 template.

* dotnet/corefxdotnet/corefx#34200 Normalize use of `this` in Html*RawTextWriter.

* dotnet/corefxdotnet/corefx#34200 Fix leading whitespace/bug #

* dotnet/corefxdotnet/corefx#34200 Add warning to generate files about modifying them.

Also fix the oversight that XmlRawTextWriterGeneratorAsync.ttinclude didn't
use the newer RawTextWriter.ttinclude abstraction.

* dotnet/corefxdotnet/corefx#34200 Make code generation produce better formatted code.

Commit migrated from https://github.com/dotnet/corefx/commit/a0e5f73d0c0876840bc4f3745e1ca8385a91cfcf