[BasicAA] Handle passthru calls in isEscapeSource()

isEscapeSource() currently considers all call return values as
escape sources. However, CaptureTracking can look through certain
calls, so we shouldn't consider these as escape sources either.

The corresponding CaptureTracking code is:
https://github.com/llvm/llvm-project/blob/7c9a3825b8420f5d37c5bb8919a9e46684a87089/llvm/lib/Analysis/CaptureTracking.cpp#L332-L333

Differential Revision: https://reviews.llvm.org/D128444