[analyzer] Don't invalidate globals when there's no call involved.

This fixes some mistaken condition logic in RegionStore that caused
global variables to be invalidated when /any/ region was invalidated,
rather than only as part of opaque function calls. This was only
being used by CStringChecker, and so users will now see that strcpy()
and friends do not invalidate global variables.

Also, add a test case we don't handle properly: explicitly-assigned
global variables aren't being invalidated by opaque calls. This is
being tracked by <rdar://problem/13464044>.

llvm-svn: 177572