Avoid calling report_fatal_error in the destructor of raw_fd_ostream

when saving a module timestamp file

This commit doesn't include a test as it requires a test that reproduces
a file write/close error that couldn't really be constructed artificially.

rdar://31860650

Differential Revision: https://reviews.llvm.org/D33357

llvm-svn: 304538

clang/lib/Serialization/ASTReader.cpp

@@ -3697,6 +3697,8 @@ static void updateModuleTimestamp(ModuleFile &MF) {
   if (EC)
     return;
   OS << "Timestamp file\n";
+  OS.close();
+  OS.clear_error(); // Avoid triggering a fatal error.
 }
 
 /// \brief Given a cursor at the start of an AST file, scan ahead and drop the