[test] Fix test dependencies when using installed tools

Use the LLVM_UTILS_PROVIDED variable to determine whether test tool
dependencies should be exposed for clang-tools-extra tests. If clang is
being built stand-alone and LLVM test tools (FileCheck, count and not)
are installed, the top-level CMakeLists.txt of clang sets this variable
to indicate that they will not be built as a part of this build,
and therefore no dependencies should be emitted for them. This fixes
the dependency errors when building clang stand-alone with tests
enabled.

Differential Revision: https://reviews.llvm.org/D29851

llvm-svn: 297806