Make TestTopLevelExprs more robust in face of linker GC

Summary:
This test was failing in various configurations on linux in a fairly
unpredictible way. The success depended on whether the c++ abi library
was linked in statically or not and how well was the linker able to
strip parts of it. This introduces additional code to the "dummmy" test
executable, which ensures that all parts of the library needed to
evaluate the expressions are always present.

Reviewers: clayborg

Subscribers: srhines, tatyana-krasnukha, davide, lldb-commits

Differential Revision: https://reviews.llvm.org/D39727

llvm-svn: 317678