[DebugInfoMetadata] Move main subprogram DIFlag into DISPFlags

Moving subprogram specific flags into DISPFlags makes IR code more readable.
In addition, we provide free space in DIFlags for other
'non-subprogram-specific' debug info flags.

Patch by Djordje Todorovic.

Differential Revision: https://reviews.llvm.org/D59288

llvm-svn: 356454