This patch drops the debug info for all DISubprograms that are
(a) not attached to an llvm::Function and
(b) not indirectly reachable via inline scopes from any surviving Function and
(c) not reachable from a type (i.e.: member functions).
Background: I'm currently working on a patch to reverse the pointers between DICompileUnit and DISubprogram (for more info check Duncan's RFC on lazy-loading of debug info metadata http://lists.llvm.org/pipermail/llvm-dev/2016-March/097419.html). The idea is to remove the list of subprograms from DICompileUnit and instead point to the owning compile unit from each DISubprogram. After doing this all DISubprograms fulfilling the above criteria will be implicitly dropped unless we go through an extra effort to preserve them. Since there is not much value in preserving DISubprograms that have been optimized away entirely (as in: not even inlined) I'm proposing to just drop them entirely and save some space.