Bug: invalid index store directory when using `swift.split_derived_files_generation`

Description

Noticed this internally and managed to repro in main => 79f26ec1315531a8459fe95934440cb3ca3c16df.

In the indexing logs I see this failure and confirmed that the .indexstore directory is not present on disk indeed:

Confirmed that the issue happens only if

common --features=swift.split_derived_files_generation

is present in the .bazelrc file.

Also noticed that this seems related to mixed_language_library since the issue goes away if I force the failing target to be a swift_library instead.

Last, it's not clear to me if this is a harmless failure or not as I see normal indexing behaviour (auto-complete, jump to def) working normally even with the .indexstore directory being non-existent.

Reproduction steps

• Checkout main => 79f26ec1315531a8459fe95934440cb3ca3c16df
• cd examples/integration
• Add common --features=swift.split_derived_files_generation to examples/integration/.bazelrc
• bazel clean --expunge
• bazel run :xcodeproj-sim_arm64
• open Integration.xcodeproj
• Observe the indexing logs

Expected behavior

No indexing errors in the logs

rules_xcodeproj version

79f26ec1315531a8459fe95934440cb3ca3c16df

Xcode version

26.0.1

Bazel version

repro-ed in both 7.x and 8.x

rules_apple version

4.0.1

rules_swift version

3.1.1

Additional information

No response

Reproduce from examples/integration/.bazelrc using common --features=swift.split_derived_files_generation, then run bazel run :xcodeproj-sim_arm64 and inspect the indexing logs. Compare the mixed_language_library path with forcing the target to swift_library; done means the generated Xcode project produces no invalid index store directory errors.