[clang] [unitttests] Fix linking Basic test to LLVMTestingSupport

[clang] [unitttests] Fix linking Basic test to LLVMTestingSupport

Link BasicTests via explicit target_link_libraries() rather than
clang_target_link_libraries() in order to fix linking when building
clang against libclang-cpp.  The latter requires all listed libraries
to be part of libclang-cpp and omits them if libclang-cpp is used.
However, LLVMTestingSupport is not part of libclang-cpp, so omitting it
causes undefined symbols.  Link to the library explicitly to follow suit
with the 7 other unittest programs.

Differential Revision: https://reviews.llvm.org/D115580