[clang] Add DISuprogram and DIE for a func decl

Attach a unique DISubprogram to a function declaration that will be
used for call site debug info.

([7/13] Introduce the debug entry values.)

Co-authored-by: Ananth Sowda <asowda@cisco.com>
Co-authored-by: Nikola Prica <nikola.prica@rt-rk.com>
Co-authored-by: Ivan Baev <ibaev@cisco.com>

Differential Revision: https://reviews.llvm.org/D60714

llvm-svn: 364502