Only track the last line info in RunContext

There were only two places where we used the full `AssertionInfo`
instance in `m_lastAssertionInfo`:
1) when reporting unexpected exception from running a test case
2) when reporting fatal error

because in those two places we do not have access to a real
instance of `AssertionInfo`, but we still need to send one to the
reporters. As a bonus, in both of these places we were already
constructing a fake-ish assertion info, by using the last encountered
source location, but dummying out the other information.

Instead, we only keep track of the last encountered source location,
and construct the dummy `AssertionInfo` on-demand.

This finishes the set of refactoring around `m_lastAssertionInfo`
in `RunContext` and improves the performance of running assertions
by ~5% in both Debug and Release mode.

--------------

Note that this change also causes small difference in output. It
could be avoided by having an invalidation flag and tracking where
the information would be invalidated before, but the difference
includes more precise line location for unexpected errors (both
exceptions and fatals), so I prefer the new output.

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