Support for partial type EnC and moving declarations across files

[tmat]

Adds support for editing partial types, moving members across files (in between different parts of a partial type declaration), adding a file to a compilation, or moving whole type declaration to another file.

The core of the change is in inverting the approach to the analysis. Previously we had two separate passes: syntactic (top-level and statement level) and semantic. If a rude edit was found during the first one we did not run the second one. The whole analysis was entirely based on the document content except for constructor semantic analysis, which could use semantic information from other documents. The new approach performs top-level only syntactic analysis and then feeds the top-level edits directly into semantic analysis. The semantic analysis translates the syntactic edits to symbol edits and if performs statement level syntactic and semantic analysis for members with updated bodies. This allows us to compare member bodies across documents - since we can run the body analysis not only on bodies located in the document being analyzed but also bodies retrieved from the syntax references of symbols.

The new approach requires the semantic and member body analysis to be more robust and handle semantically incorrect code since we no can no longer discover many of the rude edits in the first phase (top-level) and bail. E.g. the first phase can't distinguish between a deleted member declaration and a member declaration moved to another file.

Some edits in partial classes require additional post-processing. E.g. if field initializers of a partial type located in different documents are updated the resulting semantic edit should be a single edit of each of the constructors that emits these initializers. Since we still perform and store the results of the analysis per document (for each changed document) we'll end up with the constructor updates recorded for each document. The final results must merge these results before passing it to the compiler when emitting deltas.

Fixes:
#50016
#50876

Follow-ups/potential improvements:

• It is possible to emit invalid delta if semantic info is inferred from documents that are not up to date (EnC: It is possible to emit invalid delta if semantic info is inferred from documents that are not up to date #51261)
• member moves across partial definitions might be classified as renamed and not handled correctly (EnC: member moves across partial definitions might be classified as renamed and not handled correctly #51011)
• Support moving member containing an active statement to a different file (EnC: Support moving member containing an active statement to a different file #51177)
• Adding a constructor to a type with field/property initializers that contain lambdas ([Enc] Adding a constructor to a type with field/property initializers that contain lambdas #2504)

[davidwengier]

Given the size, I mainly looked through the new code, rather than strictly the diff, but it seemed to make sense.