Add a safety check.
Make sure there's no "dangling" backtrack position when Preprocessor is destroyed.

llvm-svn: 55236
diff --git a/clang/lib/Lex/Preprocessor.cpp b/clang/lib/Lex/Preprocessor.cpp
index a4ff8bb..18b106a 100644
--- a/clang/lib/Lex/Preprocessor.cpp
+++ b/clang/lib/Lex/Preprocessor.cpp
@@ -84,6 +84,8 @@
 }
 
 Preprocessor::~Preprocessor() {
+  assert(BacktrackPositions.empty() && "EnableBacktrack/Backtrack imbalance!");
+
   // Free any active lexers.
   delete CurLexer;