[LLVMTestingSupport] Add explicit linkage to LLVMSupport

Explicitly link LLVMTestingSupport library against LLVMSupport. This
is necessary to fix linking errors when LLVMTestingSupport is built
as a shared library (with BUILD_SHARED_LIBS=ON) and -Wl,-z,defs is used.

Differential Revision: https://reviews.llvm.org/D45408

llvm-svn: 329522