Extend type nullability qualifiers for Objective-C.

Extend type nullability qualifiers for Objective-C.

Introduce context-sensitive, non-underscored nullability specifiers
(nonnull, nullable, null_unspecified) for Objective-C method return
types, method parameter types, and properties.

Introduce Objective-C-specific semantics, including computation of the
nullability of the result of a message send, merging of nullability
information from the @interface of a class into its @implementation,
etc .

This is the Objective-C part of rdar://problem/18868820.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@240154 91177308-0d34-0410-b5e6-96231b3b80d8