Reapply r152745 (reverted in 152765) now that compiler-rt is fixed.

Original commit message:

Provide -Wnull-conversion separately from -Wconversion.

Like GCC, provide a NULL conversion to non-pointer conversion as a separate
flag, on by default. GCC's flag is "conversion-null" which we provide for
cross compatibility, but in the interests of consistency (with
-Wint-conversion, -Wbool-conversion, etc) the canonical Clang flag is called
-Wnull-conversion.

Patch by Lubos Lunak.
Review feedback by myself, Chandler Carruth, and Chad Rosier.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@152774 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/test/SemaCXX/__null.cpp b/test/SemaCXX/__null.cpp
index 1989a45..56e59c0 100644
--- a/test/SemaCXX/__null.cpp
+++ b/test/SemaCXX/__null.cpp
@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -triple x86_64-unknown-unknown %s -fsyntax-only -verify
-// RUN: %clang_cc1 -triple i686-unknown-unknown %s -fsyntax-only -verify
+// RUN: %clang_cc1 -triple x86_64-unknown-unknown %s -Wno-null-conversion -fsyntax-only -verify
+// RUN: %clang_cc1 -triple i686-unknown-unknown %s -Wno-null-conversion -fsyntax-only -verify
 
 void f() {
   int* i = __null;