Fix indexStorePath/indexDatabasePath being transposed in BSP response

[AnupamKumar-1]

Fixes swiftlang/swift-package-manager#10350

Note: the linked issue was filed against swift-package-manager, but the root cause is in swift-build's BSP server implementation, so the fix lives here.

Summary

indexStorePath and indexDatabasePath in SWBBuildServer's BSP build/initialize response were transposed:

• indexStorePath derived a sibling directory (dirname(indexDataStoreFolderPath)/index-store) that nothing ever writes to.
• indexDatabasePath returned indexDataStoreFolderPath directly — i.e. the real index store location — mislabeled as a database path.

Any BSP client that trusts indexStorePath (e.g. SourceKit-LSP with swiftPM.buildSystem: "swiftbuild") opens a valid-but-empty store and silently returns no results for cross-file queries.

Fix

• indexStorePath now returns arenaInfo.indexDataStoreFolderPath directly — the actual directory the build writes index data into.
• indexDatabasePath now derives a sibling directory (DataStoreDB, following the naming convention already used for adjacent Index.noindex paths in SWBTestSupport/Misc.swift: DataStore, PrecompiledHeaders, Build) so a client can safely create its own IndexStoreDB without colliding with the raw store.

Verification

Reproduced and confirmed with a real package, using the swiftbuild backend end-to-end:

1. Captured the real -index-store-path from a verbose swift build --build-system swiftbuild -v compile invocation.
2. Queried swift package experimental-build-server --build-system swiftbuild directly over stdio with a raw build/initialize request and confirmed the advertised paths didn't match the real one (before fix).
3. After the fix, rebuilt and re-queried: indexStorePath now matches the real store path, and .build/out/v5/{units,records} are confirmed present exactly where indexStorePath claims.
4. indexDatabasePath now points at a distinct, non-colliding sibling path.

Testing

Added indexStoreAndDatabasePathsAreNotTransposed in BuildServerTests.swift, which constructs a build request with an explicit SWBArenaInfo and asserts indexStorePath matches the real derived-data path while indexDatabasePath remains distinct from it. Verified the test fails without the fix and passes with it.