Check for consistent use of nullability type specifiers in a header.

Check for consistent use of nullability type specifiers in a header.

Adds a new warning (under -Wnullability-completeness) that complains
about pointer, block pointer, or member pointer declarations that have
not been annotated with nullability information (directly or inferred)
within a header that contains some nullability annotations. This is
intended to be used to help maintain the completeness of nullability
information within a header that has already been audited.

Note that, for performance reasons, this warning will underrepresent
the number of non-annotated pointers in the case where more than one
pointer is seen before the first nullability type specifier, because
we're only tracking one piece of information per header. Part of
rdar://problem/18868820.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@240158 91177308-0d34-0410-b5e6-96231b3b80d8