Fix PR6360. It's easy for a stub's address to escape to user code, so we can't

just count references to it from JIT output to decide when to destroy it.  This
patch waits to destroy the JIT's memory of a stub until the Function it refers
to is destroyed.  External function stubs and GVIndirectSyms aren't destroyed
until the JIT itself is.

llvm-svn: 97737