[SourceKit] Don't build an AST if no semantic info is requested
Currently, we were building an AST on document open or edit even if
- `key_enablesyntaxmap` = 0
- `key_enablesubstructure` = 0
- `key_enablediagnostics` = 0 and
- syntax tree transfer mode is off
In those cases we were just ignoring the result.
If all of the options are 0, don’t build an AST.
rdar://85847659
