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