Fix debug build of unit tests

Summary:
It turns out listing each library twice is not enough to resolve all
references in a debug build on linux - a number of executables fails to
link with random symbols missing. Increasing the number to three seems
to be enough. The choice of lldbCore to set the multiplicity on is
somewhat arbitrary, but it seems fitting, as it is the biggest layering
transgressor.

Reviewers: beanz

Subscribers: mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D29888

llvm-svn: 295189