[AST] Adapt Polly to AnalysisSetTracker changes. NFC.

The method AliasSetTracker::getAliasSetForPointer was removed and replaced by AliasSetTracker::getAliasSetFor for the restructuring in r339930. 

Since Polly uses AliasSetTracker::getAliasSetForPointer, a temporary fix has been committed in r339937 with a comment:

     Can someone from polly please migrate usage and then delete the wrapper?

This commit is doing exactly that.

llvm-svn: 340072