[CAS] Exposing the CAS creating API instead of the CAS

Expose CAS creating APIs to build system, instead of the created CAS.
This allows the build system to create a different CAS if needed.

Author: cachemeifyoucan

Sources/SwiftDriver/Driver/Driver.swift

@@ -275,7 +275,7 @@ public struct Driver {
   let useClangIncludeTree: Bool
 
   /// CAS instance used for compilation.
-  public var cas: SwiftScanCAS? = nil
+  var cas: SwiftScanCAS? = nil
 
   /// Is swift caching enabled.
   lazy var isCachingEnabled: Bool = {

Sources/SwiftDriver/ExplicitModuleBuilds/InterModuleDependencies/InterModuleDependencyOracle.swift

@@ -165,7 +165,7 @@ public class InterModuleDependencyOracle {
     return diags.isEmpty ? nil : diags
   }
 
-  @_spi(Testing) public func createCAS(pluginPath: AbsolutePath?, onDiskPath: AbsolutePath?, pluginOptions: [(String, String)]) throws -> SwiftScanCAS {
+  public func createCAS(pluginPath: AbsolutePath?, onDiskPath: AbsolutePath?, pluginOptions: [(String, String)]) throws -> SwiftScanCAS {
     guard let swiftScan = swiftScanLibInstance else {
       fatalError("Attempting to reset scanner cache with no scanner instance.")
     }

Tests/SwiftDriverTests/CachingBuildTests.swift

@@ -487,10 +487,17 @@ final class CachingBuildTests: XCTestCase {
       let jobs = try driver.planBuild()
       try driver.run(jobs: jobs)
       XCTAssertFalse(driver.diagnosticEngine.hasErrors)
-      guard let cas = driver.cas else {
-        XCTFail("CAS is not available")
+
+      let dependencyOracle = InterModuleDependencyOracle()
+      let scanLibPath = try XCTUnwrap(driver.toolchain.lookupSwiftScanLib())
+      guard try dependencyOracle
+              .verifyOrCreateScannerInstance(fileSystem: localFileSystem,
+                                             swiftScanLibPath: scanLibPath) else {
+        XCTFail("Dependency scanner library not found")
         return
       }
+
+      let cas = try dependencyOracle.createCAS(pluginPath: nil, onDiskPath: casPath, pluginOptions: [])
       try checkCASForResults(jobs: jobs, cas: cas, fs: driver.fileSystem)
     }
   }