Avoid adding some decls to DeferredDeclsToEmit.

Before this patch GetOrCreateLLVMFunction would add a decl to
DeferredDeclsToEmit even when it was being called by the function trying to
emit that decl.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196753 91177308-0d34-0410-b5e6-96231b3b80d8