OK. So I've installed gdb, and rebuilt Emacs as Eli suggested. I have then imported the project to Eclipse IDE for C/C++ Developers (Oxygen Release 4.7.0). I have no trouble running the GUI from Eclipse. However, when when I try to debug it, I get the following error message:

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'Launching emacs-26.0.50.1' has encountered a problem.
Error in final launch sequence
Failed to execute MI command:
-exec-run
Error message from debugger back end:
During startup program terminated with signal ?, Unknown signal.
During startup program terminated with signal ?, Unknown signal.
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On Fri, Aug 11, 2017 at 11:47 AM, Paul Eggert <eggert@cs.ucla.edu> wrote:
Itai Berli wrote:
At any rate, gdb is not installed
on my system. I can download and install it, but I'm a little hesitant, as
I'm afraid it might clash with LLVM.

 It won't break anything; although GDB consumes LLVM's output, it does not affect what LLVM does.

You don't need to rerun autogen.sh merely because you re-ran 'configure'.