Simplify loop.  No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@116861 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Lex/Preprocessor.cpp b/lib/Lex/Preprocessor.cpp
index 9a69500..a0782aa 100644
--- a/lib/Lex/Preprocessor.cpp
+++ b/lib/Lex/Preprocessor.cpp
@@ -106,11 +106,8 @@
   }
 
   // Free any macro definitions.
-  for (MacroInfoChain *I = MIChainHead ; I ; ) {
-    MacroInfoChain *Next = I->Next;
+  for (MacroInfoChain *I = MIChainHead ; I ; I = I->Next)
     I->MI.Destroy();
-    I = Next;
-  }
 
   // Free any cached macro expanders.
   for (unsigned i = 0, e = NumCachedTokenLexers; i != e; ++i)