Adding flags for outputting absolute paths for messages

[Sharlock93]

Uses std::filesystem to create absolute paths when outputting messages. I needed this myself for easy jumps in vim/visual studio error lists. This was also requested in this issue https://github.com/KhronosGroup/glslang/issues/2717

Please let me know if I'm missing anything major or if I'm missing something in terms of contribution etiquette.

[CLAassistant]

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[Sharlock93]

You will need to add this new flag to the usage message. Also, when I test this, it does something like

~/glslang$ build/StandAlone/glslang --absolute-path Test/cppBad2.vert 
Test/cppBad2.vert
ERROR: /home/arcady/glslang/0:3: 'macro expansion' : End of input in macro b
ERROR: /home/arcady/glslang/0:3: '' : compilation terminated 
ERROR: 2 compilation errors.  No code generated.

Which does not seem like the desired result since the filename is missing.

Yeah that definitely was something I didn't fully consider when the filename is missing. Latest commit should address that, I noticed that the file name was never sort of "lowered" to the compiler only the contents were (at least from just stepping into the code) so I had to store the file name somewhere, I'm not sure if that is the best place for it so any feedback on it would be appreciated.

I can also imagine some optimization around having a big shader with a lot of errors and caching the absolute path once its been outputted the first time.