[SourceKit] Add option to index local symbols to IndexSource request

Adds optional `key.should_index_locals` flag to the SourceKit `source.request.indexsource` request.
If true, the response includes symbol info for both locals and globals.

utils/gyb_sourcekit_support/UIDs.py

@@ -214,6 +214,7 @@ UID_KEYS = [
     KEY('ExpandedMacroReplacements', 'key.expanded_macro_replacements'),
     KEY('IndexStorePath', 'key.index_store_path'),
     KEY('IndexUnitOutputPath', 'key.index_unit_output_path'),
+    KEY('ShouldIndexLocals', 'key.should_index_locals'),
 ]