[CMake] Add the way to run tests in standalone build.

1) Depend on llvm-config (configured in LLVM_CONFIG_PATH) to
get necessary LLVM source/binary directories.
2) Add basic support for running lit tests (check-xsan commands).
For now this "support" is far from what we want:
  * unit tests are not built currently.
  * lit tests use Clang/compiler-rt from LLVM build directory,
    not the host compiler or just-built compiler-rt libraries.

We should make a choice on the way we intend ti run compiler-rt lit testsuite:
a) use either Clang from LLVM build tree, or the host compiler.
b) use either just-built runtimes, or the runtimes shipped with the
host compiler.

Using just-built runtimes is tricky - we have to know where to put them, so that
Clang/GCC driver would pick them up (and not overwrite the existing runtimes).
Using a host compiler instead of Clang from LLVM build tree will give us a chance to
run lit tests under GCC (which already has support for several sanitizers).

That is, I tend to make the following choice: if we're in a standalone compiler-rt
build, use host compiler with its set of runtime libraries to run lit tests.
This will effectively decouple "make compiler-rt" and "make check-compiler-rt" in
a standalone build - the latter wouldn't invoke the former. Note that if we decide
to fix LLVM/Clang/compiler-rt build system so that it would configure/build
compiler-rt with just-built Clang (as we do in Makefile-based build), this will not
be a problem - we can add a dependency to ensure that clang/compiler-rt are rebuilt
before running compiler-rt tests.