Inconsistency how to get shader source code to debug info

In order to get the source code of the _main_ source file included in debug info, it is necessary to specify `EShMsgDebugInfo` to `TShader::parse()`. Sources which are _included_ get embedded to debug info even without `EShMsgDebugInfo`. First, this is inconsistent. Second, using `messages` _in addition to_ `glslang::SpvOptions` as way to enable source code embedded to debug info is confusing..

Start by tracing TShader::parse() and the handling of EShMsgDebugInfo alongside glslang::SpvOptions; compare how main and included sources enter debug info. Determine the intended single mechanism and verify the chosen behavior with relevant parser or SPIR-V debug-info tests.

Written by the indexing model from the issue text.