The glslang standalone command line tool does not support system includes (#include <foo>)

This should just be supported. The standalone tool's includer also just returns null, so in order for a user to even understand that the command line tool is incomplete, they'd need to look into the source. See line 127 of the StandAlone/DirStackFileIncluder.h https://github.com/KhronosGroup/glslang/blob/b3a6aa7b03c51ba976e4f4e96b1e31f77f43f312/StandAlone/DirStackFileIncluder.h#L127-L131

Start by reading StandAlone/DirStackFileIncluder.h at lines 127-131 and trace how the standalone command-line tool handles include requests. Confirm the expected behavior for #include ; done means system includes are supported or the tool clearly reports the limitation instead of silently returning null.

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