Objective-C. revert r204965. This will make

Objective-C. revert r204965. This will make
-Wselector-type-mismatch default again. After
internal discussions, we think that in most cases
it has helped our developers find hard to detect
undefined behaviors. We are going to provide a syntax
(and fix-it) to suppress the warning in remaining of
false positive cases.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205024 91177308-0d34-0410-b5e6-96231b3b80d8