Comment a wacky test case

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@123758 91177308-0d34-0410-b5e6-96231b3b80d8

test/CodeGen/enum.c

 // RUN: %clang_cc1 -triple i386-unknown-unknown %s -O3 -emit-llvm -o - | grep 'ret i32 6'
 // RUN: %clang_cc1 -triple i386-unknown-unknown -x c++ %s -O3 -emit-llvm -o - | grep 'ret i32 7'
 
+// This test case illustrates a peculiarity of the promotion of
+// enumeration types in C and C++. In particular, the enumeration type
+// "z" below promotes to an unsigned int in C but int in C++.
 static enum { foo, bar = 1U } z;
 
 int main (void)