[IR] Introduced ClosureAcceptingInstr marker interface.

* Several MRI tests were crashing because live-var analyses was
  not getting run of closures passed into BuildLambdaInstr.

  To fix this, introduced a ClosureAcceptingInstr interface which
  CallBase and BuildLambdaInstr implement. Replaced most references
  to CallBase in analyses to reference the interface instead.

  Problem solved.