[analyzer] Refine the diagnostics in the nullability checker to differentiate between nil and null

This is a big deal for ObjC, where nullability annotations are extensively
used. I've also changed "Null" -> "null" and removed "is" as this is the
pattern that Sema is using.

Differential Revision: https://reviews.llvm.org/D27600

llvm-svn: 289885
diff --git a/clang/test/Analysis/nullability.mm b/clang/test/Analysis/nullability.mm
index ac2a839..e4f13c6 100644
--- a/clang/test/Analysis/nullability.mm
+++ b/clang/test/Analysis/nullability.mm
@@ -75,7 +75,7 @@
   }
   Dummy a;
   Dummy *_Nonnull nonnull = &a;
-  nonnull = q; // expected-warning {{Null is assigned to a pointer which is expected to have non-null value}}
+  nonnull = q; // expected-warning {{Null assigned to a pointer which is expected to have non-null value}}
   q = &a;
   takesNullable(q);
   takesNonnull(q);
@@ -107,7 +107,7 @@
 
 Dummy *_Nonnull testNullReturn() {
   Dummy *p = 0;
-  return p; // expected-warning {{Null is returned from a function that is expected to return a non-null value}}
+  return p; // expected-warning {{Null returned from a function that is expected to return a non-null value}}
 }
 
 void testObjCMessageResultNullability() {
@@ -229,7 +229,7 @@
 Dummy * _Nonnull testIndirectCastNilToNonnullAndReturn() {
   Dummy *p = (Dummy * _Nonnull)0;
   // FIXME: Ideally the cast above would suppress this warning.
-  return p; // expected-warning {{Null is returned from a function that is expected to return a non-null value}}
+  return p; // expected-warning {{Null returned from a function that is expected to return a non-null value}}
 }
 
 void testInvalidPropagation() {