Xcode preferences
Turn off 'Load symbols lazily', and set to show Debugger at startup:

Build settings
For maximum compatibility with different versions of Xcode, build your app with FBtoC settings like these:

and, if possible, this (to make variable names in C easier to read):

Debugging

After the build, choose File > Debug Last App.

FBtoC sets up an Xcode project to debug your app as a "custom executable" (see Debugging Arbitrary Applications With Xcode)

All going well, you get full source-code debugging, with symbolic break-points etc.:

Reference
Xcode User Guide: Debugging
Xcode User Guide: Attaching to a Running Process