[CMake] External compiler-rt-configure requires LLVMTestingSupport when including tests

Summary:
Apparently `LLVMTestingSupport` must be built before `llvm-config` can be asked for it. Symptom with `LLVM_INCLUDE_TESTS=ON` is:
```
$ ./path/to/llvm-build/bin/llvm-config --ldflags --libs testingsupport
-L/path/to/llvm-build/lib -Wl,-search_paths_first -Wl,-headerpad_max_install_names
llvm-config: error: component libraries and shared library

llvm-config: error: missing: /path/to/llvm-build/lib/libLLVMTestingSupport.a
```

With `LLVMTestingSupport` as dependency of `compiler-rt-configure` we get the expected behavior:
```
$ ./path/to/llvm-build/bin/llvm-config --ldflags --libs testingsupport
-L/path/to/llvm-build/lib -Wl,-search_paths_first -Wl,-headerpad_max_install_names
-lLLVMTestingSupport -lLLVMSupport -lLLVMDemangle
```

Reviewers: ab, beanz

Subscribers: dberris, mgorny, erik.pilkington, llvm-commits, cfe-commits

Differential Revision: https://reviews.llvm.org/D57521

llvm-svn: 352881