Fix for r364686 - actually set symbol_is_missing_weak...

This was set in a std::function, but I was shadowing a
variable that I thought I was capturing.  Even with this bug
we were correctly not raising an error and returning an address
of 0x0.  We were not marking the symbol as weak, but apparently
the JIT didn't need that, so the test still passed.

llvm-svn: 364980