Flow the list of .editorconfig files through the compiler front end

[jasonmalinowski]

Once we have the list of .editorconfig files determined, we need to get them through the compiler APIs. The expectation is these are expressed in the same form roughly as source files:

1. They go into another input into the Csc/Vbc tasks.
2. We convert those to /editorconfig switches, which are a list of file names after that. Same as any other switched input.
3. We parse those through and still represent as files in the parsed command line options, again same as everything else.

[Pilchie]

Sadly, this editorconfig work isn't going to fit for 15.3, but we're going to continue targeting the next foundation update.

[dpoeschl]

Moving .editorconfig work to 15.later.

[agocke]

Done in #27232

[smitpatel]

@agocke - Which milestone (or VS version) did this go in? Milestone here is 16.0 but in PR it is 15.8

[agocke]

16.0 is right. I've removed the milestone from the PR.