[Symbolizer] Ignore unknown additional symbolizer markup fields

The symbolizer markup syntax is structured such that fields require only
previous fields for their interpretation; this was originally intended
to make adding new fields a natural extension mechanism for existing
elements. This codifies this into the spec and makes the behavior of the
llvm-symbolizer match. Extra fields are now warned about, but ignored,
rather than ignoring the whole element.

Reviewed By: mcgrathr

Differential Revision: https://reviews.llvm.org/D153821