[clang-doc] Build as clang_tool
[clang-doc] Build as clang_tool

Instead of as clang_executable.

Differential Revision: https://reviews.llvm.org/D59974

git-svn-id: a34e9779ed74578ad5922b3306b3d80a0c825546@357274 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang-tools-extra,llvm-mirror/clang-tools-extra,llvm-mirror/clang-tools-extra,llvm-mirror/clang-tools-extra