Add ExternalAAWrapperPass to createLegacyPMAAResults.

Our out-of-tree custom aliasing solution for the HPC# Burst compiler
here at Unity makes use of the `ExternalAAwrapperPass` infrastructure to
insert our custom aliasing resolution into the core of LLVM. This is
great for all cases except for function inlining, where because
`createLegacyPMAAResults` does not make use of `ExternalAAWrapperPass`,
when we have a definite no-alias result within a function it won't be
propagated to the calling function during inlining.

This commit just rectifies this oversight by adding the missing
dependency.

Differential Revision: https://reviews.llvm.org/D71348