Previously, `vsscanf()` would crash whenever it encountered a width
specification. Now, it consumes the width specification but does not
yet do anything with it.
...for 'long long' and 'unsigned long long', instead of reading them as
'long's and 'unsigned long's.
Also add a test for values that can only fit in (unsigned) long long.
Fixes#6096.
(...and ASSERT_NOT_REACHED => VERIFY_NOT_REACHED)
Since all of these checks are done in release builds as well,
let's rename them to VERIFY to prevent confusion, as everyone is
used to assertions being compiled out in release.
We can introduce a new ASSERT macro that is specifically for debug
checks, but I'm doing this wholesale conversion first since we've
accumulated thousands of these already, and it's not immediately
obvious which ones are suitable for ASSERT.
