We introduce a new macro called #SwiftSettings that can be used in conjunction
with a new stdlib type called SwiftSetting to control the default isolation at
the file level. It overrides the current default isolation whether it is the
current nonisolated state or main actor (when -enable-experimental-feature
UnspecifiedMeansMainActorIsolated is set).
* 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.
