missing compile options (-g -O) for files in sub-groups

[andbararm]

Describe the bug
When adding files to a cproject file in nested sub-groups like this:

  groups:
    - group: Source Files
      groups:
      - group: Main Files
        files:
        - file: ./main_AB.c

then during the build, options -g (Debug Information) and -O (Optimization) are missing. As a consequence, for such a module it is not possible to set breakpoints, due to the missing debug information.
When not using nested sub-groups like this:

  groups:
    - group: Source Files
      files:
        - file: ./main_AB.c

the named options are there. The difference can be seen in build related files, like in build.ninja or the compile_commands.json.

To Reproduce
You can use the attached project. It can be opened in uVision or Keil Studio. Then build it and check named files for the missing compiler options. Like in compile_commands.json, main_AB.c is missing -g -O0, while system_DS_CM3.c is compiled with -g -O0.
Test_Solution.zip

Expected behavior
Also when putting source files in a sub-group, it should get all the compiler options like other files not in sub groups or also from other CMSIS components used in the project.

Environment (please complete the following information):

CBuild version: 2.6.1