patch: remove the unqualified name cache for typos

[Nick Lewycky]

This patch removes Sema::UnqualifiedTyposCorrected.

I'm not sure this is the right thing to do. As far as I can tell this cache
can't be made to be correct (wouldn't we need to invalidate it every time a
new decl is declared?) and I'm not convinced it saves us much compile time.
However, I'm not confident in my understanding of the code, so please do
push back if it is beneficial.

One bug it fixes is a case where the same full-expression has the two typos
in the same identifier, and we emit the correction at the SourceRange for
the first typo twice instead of once for each place it's misspelled. This
is because the SourceRange is on the TypoCorrection, and the cache key is
an IdentifierInfo. Adding the SourceRange to the cache key wouldn't make
much sense because we wouldn't ever re-use it. (NB, I have similar feelings
about TypoCorrectionFailures which I am not touching in this patch. Why
would we do typo correction twice at the same location?)

Removing it improves typo correction quality and there are changes to the
tests (see p11.cpp, and the last two tests in typo-correction) where it
fired. Because the cutoff for typos is a function of this cache, I move
that to a flag (-fspell-checking-limit=...) and turn the limit off entirely
to merge typo-correction-pt2.cpp and typo-correction.cpp. Also turn it up
from 20 to 50 to accommodate existing tests. Without caching, we run up the
counter more quickly.

Please review!

Nick
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