Improve behavior of DebugInfoEntryMinimal::getSubprogramName() introduced in r159512.

Improve behavior of DebugInfoEntryMinimal::getSubprogramName() introduced in r159512.
To fetch a subprogram name we should not only inspect the DIE for this subprogram, but optionally inspect
its specification, or its abstract origin (even if there is no inlining), or even specification of an abstract origin.

Reviewed by Benjamin Kramer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@160365 91177308-0d34-0410-b5e6-96231b3b80d8