Build clang-headers when building clang tools.

[hokein]

Summary

clang tools require clang headers to work on real project, e.g. when we
build clangd via ninja clangd, we expect the binary can run on
real-world project (without running another command ninja clang-headers).

[sammccall]

> when we build clangd via ninja clangd, we expect the binary can run on real-world project

FWIW, I don't particularly have this expectation - I expect to be able to run the tests, and install the built binary.

But in practice this doesn't seem to slow things down much.

cmake/modules/AddClang.cmake

 endmacro(add_clang_executable)
 
 macro(add_clang_tool name)
   if (NOT CLANG_BUILD_TOOLS)
     set(EXCLUDE_FROM_ALL ON)
   endif()
 
   add_clang_executable(${name} ${ARGN})
+  add_dependencies(${name} clang-headers)
 
   if (CLANG_BUILD_TOOLS)
     if(${name} IN_LIST LLVM_DISTRIBUTION_COMPONENTS OR
         NOT LLVM_DISTRIBUTION_COMPONENTS)
       set(export_to_clangtargets EXPORT ClangTargets)
       set_property(GLOBAL PROPERTY CLANG_HAS_EXPORTS True)
     endif()