Update CMakeLists.txt snippet so that example compiles

[danzimm]

Summary

Previous to this the example didn't work out of the box, it seems some cmake config changed between when this was written and now.

[steveire]

Yes, add_llvm_executable uses target_link_libraries with PRIVATE for LLVM_PTHREAD_LIB. It is designed so that if you use the PUBLIC|PRIVATE|INTERFACE keyword with a target once, you have to do it at each location.

[smeenai]

Yep, that's my doing (rL319840). I didn't think about the documentation, whoops.

[danzimm]

@steveire can you land this for me? I don't have commit access

[steveire]

Done! Thanks for the contribution!

docs/LibASTMatchersTutorial.rst

 ::
 
       set(LLVM_LINK_COMPONENTS support)
 
       add_clang_executable(loop-convert
         LoopConvert.cpp
         )
       target_link_libraries(loop-convert
+        PRIVATE
         clangTooling
         clangBasic
         clangASTMatchers
         )
 
 With that done, Ninja will be able to compile our tool. Let's give it
 something to compile! Put the following into
 ``tools/extra/loop-convert/LoopConvert.cpp``. A detailed explanation of