Use vfs::FileSystem in ASTUnit when creating CompilerInvocation.

Summary: It used to always call into the RealFileSystem before.

Reviewers: bkramer, krasimir, klimek, bruno

Reviewed By: klimek

Subscribers: bruno, cfe-commits

Differential Revision: https://reviews.llvm.org/D34469

llvm-svn: 306549
diff --git a/clang/lib/Frontend/ASTUnit.cpp b/clang/lib/Frontend/ASTUnit.cpp
index 1f34f10..c5a911f 100644
--- a/clang/lib/Frontend/ASTUnit.cpp
+++ b/clang/lib/Frontend/ASTUnit.cpp
@@ -1638,7 +1638,7 @@
                                       &StoredDiagnostics, nullptr);
 
     CI = clang::createInvocationFromCommandLine(
-        llvm::makeArrayRef(ArgBegin, ArgEnd), Diags);
+        llvm::makeArrayRef(ArgBegin, ArgEnd), Diags, VFS);
     if (!CI)
       return nullptr;
   }