AST: fix a typo in a comment (NFC)

Fix a typo spotted by Akira!  NFC

llvm-svn: 345449
diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
index 321f4f9..b805011 100644
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -5772,7 +5772,7 @@
   /// Objective-C ABI
   ///
   ///    typedef struct __NSConstantString_tag {
-  ///      const char *isa;
+  ///      const int *isa;
   ///      int flags;
   ///      const char *str;
   ///      long length;