use legacy driver when generating index store output

Summary:
The swift-driver enforces output paths are unique, and will fail when producing index store output with:
```
SwiftDriverExecution/MultiJobExecutor.swift:207: Fatal error: multiple producers
for output /dev/null: Compiling ...
```
For now force the legacy driver for index store output.

Reviewed By: drodriguez

Differential Revision: D68978696

fbshipit-source-id: 38c94f3202abbf13cb7cf07758ceab5c63462349

prelude/apple/swift/swift_compilation.bzl

@@ -650,6 +650,7 @@ def _compile_index_store(
         index_store_output.as_output(),
         "-c",
         "-disable-batch-mode",
+        "-disallow-use-new-driver",
     ])
 
     _compile_with_argsfile(