Don't immediately delete generated doc file

Just before writing the object file. Module::deleteObjFile() deletes the
doc file too, and generating the docs happens in a prior pass before
codegen.
deleteObjFile() is already called in mars.d, after reading the source
file, before the generate-doc pass.

Fixes dmd-testsuite's runnable/test_switches.sh.

driver/codegenerator.cpp

@@ -166,7 +166,6 @@ void CodeGenerator::finishLLModule(Module *m) {
                        *global.params.bitcodeFiles);
   }
 
-  m->deleteObjFile();
   writeAndFreeLLModule(m->objfile->name->str);
 }