[SourceKit] Add optional declarations array to interface gen request (#75802)

Introduces the new DeclarationsArrayBuilder and adds it to the
EditorConsumer. Declaration info always includes a kind, offset, and
length, and includes a USR where applicable.
As the USR is already available for editor.open.interface type requests,
this doesn't compute any new information, it just exposes more of what's
there already.

utils/gyb_sourcekit_support/UIDs.py

@@ -67,6 +67,8 @@ UID_KEYS = [
     KEY('ModuleImportDepth', 'key.moduleimportdepth'),
     KEY('NumBytesToErase', 'key.num_bytes_to_erase'),
     KEY('NotRecommended', 'key.not_recommended'),
+    KEY('Declarations', 'key.declarations'),
+    KEY('EnableDeclarations', 'key.enabledeclarations'),
     KEY('Annotations', 'key.annotations'),
     KEY('SemanticTokens', 'key.semantic_tokens'),
     KEY('DiagnosticStage', 'key.diagnostic_stage'),