Cache the location of failed typo corrections so that typo correction
isn't repeatedly attempted for the same identifier at the same location.
Avoid the hard-coded limit on the number of typo corrections attempted.

Move some tests from typo-correction.cpp to typo-correction-pt2.cpp
because they were running afoul of the hard-coded limit of 20 typos
corrected. Some of the tests after it were still working due to the
limit not applying to cached corrections and in cases where a non-NULL
MemberContext is passed in to Sema::CorrectTypo.  Most of the moved tests
still passed after being moved, but the test involving "data_struct" had
only been passing because the test had exceeded that limit so a fix for
it is also included (most of the changes to ParseStmt.cpp are shared with
and originated from another typo correction impovement that was split
into a separate commit).
Don't give suggest things like function names on the left side of "=".