[analyzer] Fix crash in body farm for getter without implicit self.

[analyzer] Fix crash in body farm for getter without implicit self.

Fix a crash in body farm when synthesizing a getter for a property
synthesized for a property declared in a protocol on a class extension
that shadows a declaration of the property in a category.

In this case, Sema doesn't fill in the implicit 'self' parameter for the getter
in the category, which leads to a crash when trying to synthesize the getter
for it.

To avoid the crash, skip getter synthesis in body farm if the self parameter is
not filled int.

rdar://problem/29938138

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@291635 91177308-0d34-0410-b5e6-96231b3b80d8