* Fix mismatch on 'FixIt.Change.replace' between several application
implementations. Specifically '.replace' should includes the trivia.
* Ignore no-op 'FixIt.Change.replace{Leading|Trailing}Triviia'.
* Make ExportedSourceFile hold any Syntax as the root node
* Move `ExportedSourceFileRequest::evaluate()` to `ParseRequests.cpp`
* Pass the decl context and `GeneatedSourceFileInfo::Kind` to
`swift_ASTGen_parseSourceFile()` to customize the parsing
* Make `ExportedSourceFile` to hold an arbitrary Syntax node
* Move round-trip checking into `ExportedSourceFileRequest::evaluate()`
* Split `parseSourceFileViaASTGen` completely from C++ parsing logic
(in `ParseSourceFileRequest::evaluate()`)
* Remove 'ParserDiagnostics' experimental feature: Now that we have
ParserASTGen mode which includes the swift-syntax parser diagnostics.
