[docs] Add the compiler-rt requirement to the test suite doc

[MaskRay]

I think the requirements are better documented by using the actual paths, e.g. MicroBenchmarks/XRay/ in this case.
Add @Meinersbur for some ideas how per-project requirements should be organized.

[mtrofin]

feedback

[Meinersbur]

With XRay basically unmaintained, I would love to see the compiler-rt requirement to go away. At least, its availability could be autodetected. That is, if unavailable, one could see that the X-Ray tests are disabled in the cmake output.

Until then I think the note could be helpful.

llvm/docs/TestSuiteGuide.md

47–48

It might be preferable to add it to LLVM_ENABLE_RUNTIMES instead.

[Meinersbur]

Patch to disable X-Ray tests if compiled without compiler-rt: D106415

[mtrofin]

LLVM_ENABLE_RUNTIMES instead of _PROJECTS

[mtrofin]

updated note

llvm/docs/TestSuiteGuide.md

47–48

done. also reworded the note in light of D106415.

[Meinersbur]

LGTM

In D101467#2724147, @MaskRay wrote:

Add @Meinersbur for some ideas how per-project requirements should be organized.

We don't have a requirements list in the docs yet. I think it is going beyond the scope of this patch to create one now.

llvm/docs/TestSuiteGuide.md

3. Create a build directory and use CMake to configure the suite. Use the
    ```bash
    % mkdir test-suite-build
    % cd test-suite-build
    % cmake -DCMAKE_C_COMPILER=<path to llvm build>/bin/clang \
            -C../test-suite/cmake/caches/O3.cmake \
            ../test-suite
    ```
 
+**NOTE!** if you are using your built clang, and you want to build and run the
+MicroBenchmarks/XRay microbenchmarks, you need to add `compiler-rt` to your
+`LLVM_ENABLE_RUNTIMES` cmake flag.
+
 4. Build the benchmarks:
 
    ```text
    % make
    Scanning dependencies of target timeit-target
    [  0%] Building C object tools/CMakeFiles/timeit-target.dir/timeit.c.o
    [  0%] Linking C executable timeit-target
    ...