[AA] A global cannot escape through nocapture/nocallback call.

When an internal global is passed to a 'nocallback' call as
a 'nocapture' pointer, it cannot escape through this call and
be indirectly referenced in this module.
So it must not alias with any pointer in the module.

This may provide some remedy for Fortran module-private array descriptors
that are usually passed by address to some runtime functions
(e.g. to allocation/deallocation functions). In general, a good aliasing
information derived from Fortran language rules would solve the same issue,
but I think this change may be beneficial as-is (given that nocapture,
nocallback attributes are properly set).

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D138336