The `-index-store-path` option is an Apple extension not available in our
Clang. Make it opt-in by default.

(It is assumed the `target.type == test` check is for Xcode support, because
there is no evidence of it in swift-corelibs-xctest.)

--- a/Sources/Build/BuildPlan/BuildPlan.swift
+++ b/Sources/Build/BuildPlan/BuildPlan.swift
@@ -111,14 +111,7 @@
         case .off:
             addIndexStoreArguments = false
         case .auto:
-            if configuration == .debug {
-                addIndexStoreArguments = true
-            } else if target.type == .test {
-                // Test discovery requires an index store for the test target to discover the tests
-                addIndexStoreArguments = true
-            } else {
                 addIndexStoreArguments = false
-            }
         }
 
         if addIndexStoreArguments {