fix(debuginfo): Skip eliminated functions in linked objects

[jan-auer]

Fixes an issue that indirectly lead to "inline parent offset too large for symcache file format" errors by reintroducing checks for functions with a DW_AT_low_pc of 0.

In regular shared objects or executables, such functions have been eliminated by the linker. As the primary use case of this library is to symbolicate stack traces, yielding such functions from the function iterator is not desirable. Also, the virtual address of the functions is no longer valid and implicitly overlaps with other eliminated functions. At least in the default behavior, they should not be included.

Removing the checks introduced a 10% performance regression on large ELF symbols. I did not notice this on my initial tests, since I missed the fact that this case is only triggered by the GNU linker. If it weren't for the symcache bug, the performance hit alone would be bearable.

Now, for relocatable objects it may make sense to include the functions. This means, there are two options:

1. Adding a method on DwarfFunctionIterator that allows to override the check. This has the disadvantage that it would not be exposed on the trait or on ObjectFunctionIterator. A possible interface could be:

for function in debug_session.functions().include_eliminated(true) { ... }

2. The DWARF implementation could detect that this is a Relocatable, and include the function by default and hide them otherwise. This would mean that the iterator magically does the right thing. Also, this comes closest to the original intention of the previously removed code comment.

Since I am gravitating to option 2, this is what is implemented in this patch.

Partially reverts #173
cc @nnethercote

[mitsuhiko]

Just the bool thing. Logic seems sensible.

[nnethercote]

I think this will preserve the behaviour I need for my use case involving .o files. Thanks for keeping it in mind!