[CodeGen] clarify a comment; NFC

Prompted by discussion on https://reviews.llvm.org/D78148.

clang/lib/CodeGen/CodeGenModule.cpp

@@ -2796,8 +2796,8 @@ bool CodeGenModule::shouldEmitFunction(GlobalDecl GD) {
 
   // PR9614. Avoid cases where the source code is lying to us. An available
   // externally function should have an equivalent function somewhere else,
-  // but a function that calls itself is clearly not equivalent to the real
-  // implementation.
+  // but a function that calls itself through asm label/`__builtin_` trickery is
+  // clearly not equivalent to the real implementation.
   // This happens in glibc's btowc and in some configure checks.
   return !isTriviallyRecursive(F);
 }