Add a parameter to LoadFromASTFile that accepts a file system and defaults to the real file-system.

clang/include/clang/Frontend/ASTUnit.h

public:
   static std::unique_ptr<ASTUnit>
   LoadFromASTFile(const std::string &Filename,
                   const PCHContainerReader &PCHContainerRdr, WhatToLoad ToLoad,
                   IntrusiveRefCntPtr<DiagnosticsEngine> Diags,
                   const FileSystemOptions &FileSystemOpts,
                   bool UseDebugInfo = false, bool OnlyLocalDecls = false,
                   CaptureDiagsKind CaptureDiagnostics = CaptureDiagsKind::None,
                   bool AllowASTWithCompilerErrors = false,
-                  bool UserFilesAreVolatile = false);
+                  bool UserFilesAreVolatile = false,
+                  IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS =
+                      llvm::vfs::getRealFileSystem());
 
 private:
   /// Helper function for \c LoadFromCompilerInvocation() and
   /// \c LoadFromCommandLine(), which loads an AST from a compiler invocation.
   ///
   /// \param PrecompilePreambleAfterNParses After how many parses the preamble
   /// of this translation unit should be precompiled, to improve the performance
   /// of reparsing. Set to zero to disable preambles.

clang/lib/Frontend/ASTUnit.cpp

Diags->setClient(new FilterAndStoreDiagnosticConsumer(
         CaptureDiagnostics != CaptureDiagsKind::AllWithoutNonErrorsFromIncludes));
 }
 
 std::unique_ptr<ASTUnit> ASTUnit::LoadFromASTFile(
     const std::string &Filename, const PCHContainerReader &PCHContainerRdr,
     WhatToLoad ToLoad, IntrusiveRefCntPtr<DiagnosticsEngine> Diags,
     const FileSystemOptions &FileSystemOpts, bool UseDebugInfo,
     bool OnlyLocalDecls, CaptureDiagsKind CaptureDiagnostics,
-    bool AllowASTWithCompilerErrors, bool UserFilesAreVolatile) {
+    bool AllowASTWithCompilerErrors, bool UserFilesAreVolatile,
+    IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS) {
   std::unique_ptr<ASTUnit> AST(new ASTUnit(true));
 
   // Recover resources if we crash before exiting this method.
   llvm::CrashRecoveryContextCleanupRegistrar<ASTUnit>
     ASTUnitCleanup(AST.get());
   llvm::CrashRecoveryContextCleanupRegistrar<DiagnosticsEngine,
     llvm::CrashRecoveryContextReleaseRefCleanup<DiagnosticsEngine>>
     DiagCleanup(Diags.get());
 
   ConfigureDiags(Diags, *AST, CaptureDiagnostics);
 
   AST->LangOpts = std::make_shared<LangOptions>();
   AST->OnlyLocalDecls = OnlyLocalDecls;
   AST->CaptureDiagnostics = CaptureDiagnostics;
   AST->Diagnostics = Diags;
-  IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS =
-      llvm::vfs::getRealFileSystem();
   AST->FileMgr = new FileManager(FileSystemOpts, VFS);
   AST->UserFilesAreVolatile = UserFilesAreVolatile;
   AST->SourceMgr = new SourceManager(AST->getDiagnostics(),
                                      AST->getFileManager(),
                                      UserFilesAreVolatile);
   AST->ModuleCache = new InMemoryModuleCache;
   AST->HSOpts = std::make_shared<HeaderSearchOptions>();
   AST->HSOpts->ModuleFormat = std::string(PCHContainerRdr.getFormat());