[SwiftASTContext] Remove abort and add FIXME
[SwiftASTContext] Remove abort and add FIXME

There was an abort() in CanPassInRegisters to figure out which tests
where hitting this code. At least TestSwiftReturns and TestSwiftStepping
were failing because of this Darwin. I've removed the abort and replaced
it with a FIXME.

apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb