Header generator: Enable to find latest Clang
Header generator: Enable to find latest Clang

This also allows to install `libclang1` instead of `libclang1-dev` as
one of the build dependencies on Ubuntu/Debian.

Signed-off-by: Christian Blichmann <64f74475e4b4dcd49beccaa15e0fb76629e72aed@google.com>

google/sandboxed-api,google/sandboxed-api,google/sandboxed-api,google/sandboxed-api