[CodeGen] Attach no-builtin attributes to function definitions with no Decl

When using -fno-builtin[-<name>], we don't attach the IR attributes to
function definitions with no Decl, like the ones created through
`CreateGlobalInitOrDestructFunction`.

This results in projects using -fno-builtin or -ffreestanding to start
seeing symbols like _memset_pattern16.

The fix changes the behavior to always add the attribute if LangOptions
requests it.

Differential Revision: https://reviews.llvm.org/D73495