[CodeGen] Restrict isTriviallyRecursive to predefined lib functions forwarding to lib functions

isTriviallyRecursive is only supposed to guard functions part of the
implementation.

This fixes PR23953.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@240735 91177308-0d34-0410-b5e6-96231b3b80d8