Handle '#line' in '-E' that has an empty file name.  Fixes <rdar://problem/8439412>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@114142 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Frontend/PrintPreprocessedOutput.cpp b/lib/Frontend/PrintPreprocessedOutput.cpp
index d0aef29..00ab3c3 100644
--- a/lib/Frontend/PrintPreprocessedOutput.cpp
+++ b/lib/Frontend/PrintPreprocessedOutput.cpp
@@ -164,11 +164,11 @@
   // Emit #line directives or GNU line markers depending on what mode we're in.
   if (UseLineDirective) {
     OS << "#line" << ' ' << LineNo << ' ' << '"';
-    OS.write(&CurFilename[0], CurFilename.size());
+    OS.write(CurFilename.data(), CurFilename.size());
     OS << '"';
   } else {
     OS << '#' << ' ' << LineNo << ' ' << '"';
-    OS.write(&CurFilename[0], CurFilename.size());
+    OS.write(CurFilename.data(), CurFilename.size());
     OS << '"';
 
     if (ExtraLen)