Add modules downloaded by ModuleCache to the global ModuleList

Adding the modules to the global module list eleminate issues in the
case when a module is unloaded from the target but some object (e.g.
breakpoint) still referencing them with weak pointers. It also speeds
up the case when we load, unload, load the same shared library because
the global module cache will keep the parsed debug info around between
the 2 load (this scenario happens for some code on android).

Differential revision: http://reviews.llvm.org/D15415

llvm-svn: 255260