[objcmt] When whitelisting the headers we want to modify, allow changing the

[objcmt] When whitelisting the headers we want to modify, allow changing the
the ObjC implementation declarations, just don't change implementations for
classes that are not in the whitelisted headers.

For example, if we change a method to return 'instancetype' we should also
update the method definition in the implementation.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197075 91177308-0d34-0410-b5e6-96231b3b80d8