[IR] Add DISuprogram and DIE for a func decl
A unique DISubprogram may be attached to a function declaration used for
call site debug info.
([6/13] Introduce the debug entry values.)
Co-authored-by: Ananth Sowda <email@example.com>
Co-authored-by: Nikola Prica <firstname.lastname@example.org>
Co-authored-by: Ivan Baev <email@example.com>
Differential Revision: https://reviews.llvm.org/D60713