Replace addEarlyAsPossiblePasses callback with adjustPassManager

This change introduces adjustPassManager target callback giving a
target an opportunity to tweak PassManagerBuilder before pass
managers are populated.

This generalizes and replaces addEarlyAsPossiblePasses target
callback. In particular that can be used to add custom passes to
extension points other than EP_EarlyAsPossible.

Differential Revision: https://reviews.llvm.org/D28336

llvm-svn: 293189