Introduce the clang pragmas "assume_nonnull begin" and "assume_nonnull
end" in which we make default assumptions about the nullability of many
unannotated pointers:

  - Single-level pointers are inferred to __nonnull
  - NSError** in a (function or method) parameter list is inferred to
    NSError * __nullable * __nullable.
  - CFErrorRef * in a (function or method) parameter list is inferred
    to CFErrorRef __nullable * __nullable.
  - Other multi-level pointers are never inferred to anything.

Implements rdar://problem/19191042.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@240156 91177308-0d34-0410-b5e6-96231b3b80d8