[CMake] Fix standalone builds: make dependency to LLVM's `count` conditional

Summary:
In standalone builds of LLDB we currently have no target `count` that tests can depend on. This may be fixed in the future by exporting the target from LLVM similar to targets llvm-config, dsymutil and others.

In-tree build with this patch:
```
    $ ninja -t query tools/lldb/lit/check-lldb-lit
    tools/lldb/lit/check-lldb-lit:
    input: phony
        tools/lldb/lit/CMakeFiles/check-lldb-lit
        bin/FileCheck
        bin/clang
        bin/count
        bin/darwin-debug
        bin/debugserver
        bin/dsymutil
        bin/llc
        bin/lldb
        bin/lldb-mi
        bin/lldb-server
        bin/lldb-test
        bin/llvm-config
        bin/llvm-mc
        bin/llvm-objcopy
        bin/not
        bin/yaml2obj
        lib/liblldb.dylib
        projects/libcxx/lib/cxx
        tools/lldb/unittests/LLDBUnitTests
    outputs:
        tools/lldb/test/check-lldb
        check-lldb-lit
```

Standalone build with this patch:
```
    $ ninja -t query lit/check-lldb-lit
    lit/check-lldb-lit:
    input: phony
        lit/CMakeFiles/check-lldb-lit
        bin/darwin-debug
        bin/debugserver
        bin/lldb
        bin/lldb-mi
        bin/lldb-server
        bin/lldb-test
        lib/liblldb.dylib
        unittests/LLDBUnitTests
    outputs:
        test/check-lldb
        check-lldb-lit
```

Reviewers: davide, aprantl, JDevlieghere, alexshap

Reviewed By: davide

Subscribers: mgorny, lldb-commits, #lldb

Differential Revision: https://reviews.llvm.org/D56389

llvm-svn: 350538