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.

llvm-svn: 240596
diff --git a/clang/lib/Sema/SemaCodeComplete.cpp b/clang/lib/Sema/SemaCodeComplete.cpp
index ebb6bbc..ff6a3ee 100644
--- a/clang/lib/Sema/SemaCodeComplete.cpp
+++ b/clang/lib/Sema/SemaCodeComplete.cpp
@@ -1343,9 +1343,9 @@
   }
 
   // Nullability
-  Results.AddResult(Result("__nonnull", CCP_Type));
-  Results.AddResult(Result("__null_unspecified", CCP_Type));
-  Results.AddResult(Result("__nullable", CCP_Type));
+  Results.AddResult(Result("_Nonnull", CCP_Type));
+  Results.AddResult(Result("_Null_unspecified", CCP_Type));
+  Results.AddResult(Result("_Nullable", CCP_Type));
 }
 
 static void AddStorageSpecifiers(Sema::ParserCompletionContext CCC,