Free the constants that have no uses in ~LLVMContext.

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).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@80590 91177308-0d34-0410-b5e6-96231b3b80d8