llvm-c: Add LLVMDeleteInstruction to fix a test issue

Not deleting the loose instruction with metadata associated to it causes
an assertion when the LLVMContext is destroyed. This was previously
hidden by the fact that llvm-c-test does not call LLVMShutdown. The
planned removal of ManagedStatic exposed this issue.

Differential Revision: https://reviews.llvm.org/D129114