Avoid double deletion in Clang driver.

Llvm module object is shared between CodeGenerator and BackendConsumer,
in both classes it is stored as std::unique_ptr, which is not a good
design solution and can cause double deletion error. Usually it does
not occur because in BackendConsumer::HandleTranslationUnit the
ownership of CodeGenerator over the module is taken away. If however
this method is not called, the module is deleted twice and compiler crashes.

As the module owned by BackendConsumer is always the same as CodeGenerator
has, pointer to llvm module can be removed from BackendGenerator.

Differential Revision: http://reviews.llvm.org/D15450

llvm-svn: 261222