[CSSimplify] Don't ignore type mismatches in "destination" of a mutating operator
If argument is l-value but the underlying types didn't line up
let's record a fix but make it a higher impact than a "source"
to make sure that an overload that has a mismatch on the other
side ("source" side) is always preferred because it supports
convertions and easier to fix comparing to `inout` parameter.
Resolves: rdar://139217343
(cherry picked from commit 76e778e468d9a352a51f914bcd6e27e7d8dcefde)