Build clang-headers when building clang tools.

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`).

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: mgorny, ilya-biryukov, ioeric, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D52714

llvm-svn: 343459

clang/cmake/modules/AddClang.cmake

@@ -131,6 +131,7 @@ macro(add_clang_tool name)
   endif()
 
   add_clang_executable(${name} ${ARGN})
+  add_dependencies(${name} clang-headers)
 
   if (CLANG_BUILD_TOOLS)
     if(${name} IN_LIST LLVM_DISTRIBUTION_COMPONENTS OR