Make -Wunused warning rules more consistent.

This patch does a few different things.

This patch improves unused var diags for const vars: we no longer
unconditionally suppress diagnostics for const vars, instead only suppressing
the diagnostic when the declaration appears to be useful.

This patch also makes us more consistently use whether a variable/function
is declared in the main file to suppress diagnostics where appropriate.

Fixes <rdar://problem/14907887>.