[CursorInfo] Add a synthesized field

Mark implicit declarations with a `synthesized` field, since while we
still want them to have a cursor info result (eg. for their USR to find
possible overrides), it's likely that clients will want to treat them
differently to results that have a real location in source.

An alternative could be to remove the location entirely, but:
  - the module name would also have to be removed to prevent wasted
    lookups in the generated interface
  - having the location could still be useful as the location that
    caused the synthesized declaration (though at the moment only
    synthesized initializers have a location)

Resolves rdar://84990655

tools/SourceKit/lib/SwiftLang/SwiftSourceDocInfo.cpp

@@ -1114,6 +1114,8 @@ fillSymbolInfo(CursorSymbolInfo &Symbol, const DeclInfo &DInfo,
 
   Symbol.IsSystem = DInfo.VD->getModuleContext()->isSystemModule();
   Symbol.IsDynamic = DInfo.IsDynamic;
+  Symbol.IsSynthesized = DInfo.VD->isImplicit();
+
   Symbol.ParentNameOffset = getParamParentNameOffset(DInfo.VD, CursorLoc);
 
   return llvm::Error::success();