Finish cross-process JIT work, and clean up previous work.

1. When the JIT is asked to remove a function, updating it's
   mapping to 0, we invalidate any function stubs used only 
   by that function.  Now, also invalidate the JIT's mapping
   from the GV the stub pointed to, to the address of the GV.

2. When dlsym stubs for cross-process JIT are enabled, do not
   abort just because a named function cannot be found in the
   JIT's process.

3. Fix various assumptions about when it is ok to use the lazy
   resolver when non-lazy JITing is enabled.

llvm-svn: 66324