Free the constants that have no uses in ~LLVMContext.

This fixes leaks from LLVMContext in multithreaded apps.
Since constants are only deleted if they have no uses, it is safe to not delete
a Module on shutdown, as many single-threaded tools do.
Multithreaded apps should however delete the Module before destroying the
Context to ensure that there are no leaks (assuming they use a different context
for each thread).

llvm-svn: 80590