Replace __double_underscored type nullability qualifiers with _Uppercase_underscored

Addresses a conflict with glibc's __nonnull macro by renaming the type
nullability qualifiers as follows:

  __nonnull -> _Nonnull
  __nullable -> _Nullable
  __null_unspecified -> _Null_unspecified

This is the major part of rdar://problem/21530726, but does not yet
provide the Darwin-specific behavior for the old names.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@240596 91177308-0d34-0410-b5e6-96231b3b80d8