[clang] Add install targets for API headers

Add an install target for clang's API headers, which allows them to be
included in distributions. The install rules already existed, but they
lacked a component and a target, making them only accessible via a full
install. These headers are useful for writing clang-based tooling, for
example. They're the clang equivalent to the llvm-headers target and
complement the clang-libraries target.

Differential Revision: https://reviews.llvm.org/D58317

llvm-svn: 355853