Doc patch for the branch only.

   for source line correspondence.</p>

<p>Each input file is encoded as a separate compile unit in LLVM debugging

   information output. However, many target specific tool chains prefer to

   encode only one compile unit in an object file. In this situation, the LLVM

   code generator will include debugging information entities in the compile

   unit that is marked as main compile unit. The code generator accepts maximum

   one main compile unit per module. If a module does not contain any main

   compile unit then the code generator will emit multiple compile units in the

   output object file.</p>

   information output. However the code generator emits only one compile unit,

   marked as main compile unit, in an object file's debugging information section.

   Most of the, if not all, target specific tool chains expect only one compile 

   unit entry per object file. </p>

</div>

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