[CodeGen] remove stale comment; NFC

The libm functions with LLVM intrinsic twins were moved above this blob with:
https://reviews.llvm.org/rL319593

llvm-svn: 319618
diff --git a/clang/lib/CodeGen/CGBuiltin.cpp b/clang/lib/CodeGen/CGBuiltin.cpp
index 76b6db1..962f5d8 100644
--- a/clang/lib/CodeGen/CGBuiltin.cpp
+++ b/clang/lib/CodeGen/CGBuiltin.cpp
@@ -1028,7 +1028,7 @@
   }
 
   switch (BuiltinID) {
-  default: break;  // Handle intrinsics and libm functions below.
+  default: break;
   case Builtin::BI__builtin___CFStringMakeConstantString:
   case Builtin::BI__builtin___NSStringMakeConstantString:
     return RValue::get(ConstantEmitter(*this).emitAbstract(E, E->getType()));