Avoid invalidating successfully loaded module files

Successfully loaded module files may be referenced in other
ModuleManagers, so don't invalidate them. Two related things are fixed:

1) I thought the last module in the manager was always the one that
failed, but it isn't.  So check explicitly against the list of
vetted modules from ReadASTCore.

2) We now keep the file descriptor of pcm file open, which avoids the
possibility of having two different pcms for the same module loaded when
building in parallel with headers being modified during a build.

<rdar://problem/16835846>