[fidlc] Deduplicate fidlc arguments by behavior and path

Previously, a second flag of the same type will silently override the
first e.g. --table A.cc --table B.cc would result in fidlc emitting
the coding tables into B.cc, and create an empty file at A.cc.

This patch changes fidlc to support multiple outputs of the same type.
It is a necessary step for use cases in zircon, where one could have
one of 1. only C binding, 2. only llcpp, 3. C + llcpp.

TEST: 1. Manually run fidlc to make sure it supports multiple outputs.
2. Tested with llcpp in the next patch in the stack.

Change-Id: Ia4d18998752c86d28a66c7a514bf43e44bb9da84