[CAS] Use IncludeTreeFileList instead of full CASFS for caching

Use IncludeTreeFileList instead of full feature CASFS for swift
dependency filesystem. This allows smaller CAS based VFS that is smaller
and faster. This is enabled by the CAS enabled compilation does not
need to iterate file system.

rdar://136787368

include/swift/Serialization/SerializedModuleLoader.h

@@ -265,7 +265,7 @@ public:
                         const llvm::DenseSet<clang::tooling::dependencies::ModuleID> &alreadySeenClangModules,
                         clang::tooling::dependencies::DependencyScanningTool &clangScanningTool,
                         InterfaceSubContextDelegate &delegate,
-                        llvm::TreePathPrefixMapper *mapper,
+                        llvm::PrefixMapper *mapper,
                         bool isTestableImport) override;
 };