Add 'remark' diagnostic type in 'clang'

A 'remark' is information that is not an error or a warning, but rather some
additional information provided to the user. In contrast to a 'note' a 'remark'
is an independent diagnostic, whereas a 'note' always depends on another
diagnostic.

A typical use case for remark nodes is information provided to the user, e.g.
information provided by the vectorizer about loops that have been vectorized.

This patch provides the initial implementation of 'remarks'. It includes the
actual definiton of the remark nodes, their printing as well as basic parameter
handling. We are reusing the existing diagnostic parameters which means a remark
can be enabled with normal '-Wdiagnostic-name' flags and can be upgraded to
an error using '-Werror=diagnostic-name'. '-Werror' alone does not upgrade
remarks.

This patch is by intention minimal in terms of parameter handling. More
experience and more discussions will most likely lead to further enhancements
in the parameter handling.