[llvm-mt] Do not build llvm-mt if not functional

[Meinersbur]

llvm-mt requires libxml2 to work, so do not even build it without libxml2.

CMake 3.31 and later prefer llvm-mt.exe over Microsoft's mt.exe if available and using clang-cl.exe as CMAKE_CXX_COMPILER. When CMake picks up llvm-mt.exe without libxml2, any build will fail with the message

llvm-mt: error: no libxml2

Bug report: #134237

Any test except --help already uses REQUIRES: libxml2. There is no point in having a non-functional executable. Not building llvm-mt.exe will force CMake to use Microsoft's mt.exe instead.

[zmodem]

This only seems to remove the llvm-mt dependency in llvm/test/CMakeLists.txt. I think llvm-mt will still get built, since llvm/tools/CMakeLists.txt includes llvm/tools/llvm-mt/CMakeLists.txt ?

[zmodem]

D'oh, for some reason I didn't see your change to llvm/tools/llvm-mt/CMakeLists.txt, which is the important one. Now it makes more sense :)

[llvm-ci]

LLVM Buildbot has detected a new failure on builder clangd-ubuntu-tsan running on clangd-ubuntu-clang while building llvm at step 2 "checkout".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/134/builds/16500

Here is the relevant piece of the build log for the reference

Step 2 (checkout) failure: update (failure)
git version 2.17.1
fatal: unable to access 'https://github.com/llvm/llvm-project.git/': Could not resolve host: github.com
fatal: unable to access 'https://github.com/llvm/llvm-project.git/': Could not resolve host: github.com