Allow subclassing ExternalAA

This allows testing AMDGPU alias analysis like any
other alias analysis pass. This fixes the existing
test pointlessly running opt -O3 when it really
just wants to run the one analysis.

Before there was no way to test this using -aa-eval
with opt, since the default constructed pass
is run. The wrapper subclass allows the
default constructor to pass the necessary callback.

llvm-svn: 346353