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/test/SemaObjC/nullability-arc.m b/clang/test/SemaObjC/nullability-arc.m
index 917a808..1c303e8 100644
--- a/clang/test/SemaObjC/nullability-arc.m
+++ b/clang/test/SemaObjC/nullability-arc.m
@@ -5,6 +5,6 @@
 @end
 
 // ARC qualifiers stacked with nullability.
-void accepts_arc_qualified(NSFoo * __unsafe_unretained __nonnull obj) {
+void accepts_arc_qualified(NSFoo * __unsafe_unretained _Nonnull obj) {
   accepts_arc_qualified(0); // expected-warning{{null passed to a callee that requires a non-null argument}}
 }