Fix http://llvm.org/PR4822: allow module deletion after a function has been

compiled.

When functions are compiled, they accumulate references in the JITResolver's
stub maps. This patch removes those references when the functions are
destroyed.  It's illegal to destroy a Function when any thread may still try to
call its machine code.

This patch also updates r83987 to use ValueMap instead of explicit CallbackVHs
and fixes a couple "do stuff inside assert()" bugs from r84522.

llvm-svn: 84975