Use the reserved keyword spelling of 'typeof'

No functional change intended, just a drive-by cleanup.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@243826 91177308-0d34-0410-b5e6-96231b3b80d8

test/CodeGenCXX/exceptions.cpp

 // RUN: %clang_cc1 %s -triple=x86_64-linux-gnu -emit-llvm -o - -fcxx-exceptions -fexceptions | FileCheck %s
 
-typedef typeof(sizeof(0)) size_t;
+typedef __typeof(sizeof(0)) size_t;
 
 // This just shouldn't crash.
 namespace test0 {