[clang-tidy] Add dependency on clang-headers

Currently, to be able to process a source file including e.g. stddef.h with
clang-tidy, one has to build both clang-tidy and the clang-headers target.
Since stddef.h is needed for virtually any source file, let clang-tidy depend
on clang-headers, so that it Just Works after it has been built.

http://reviews.llvm.org/D22046

llvm-svn: 274751

clang-tools-extra/clang-tidy/tool/CMakeLists.txt

@@ -5,6 +5,9 @@ set(LLVM_LINK_COMPONENTS
 add_clang_executable(clang-tidy
   ClangTidyMain.cpp
   )
+add_dependencies(clang-tidy
+  clang-headers
+  )
 target_link_libraries(clang-tidy
   clangAST
   clangASTMatchers