[analyzer] Fix false negative when pass implicit cast nil to nonnull

We should look through implicit casts before determining the type of the arguments, and only allow explicit cast to _Nonnull to suppress warning

```
void foo(NSString *_Nonnull);

foo((NSString * _Nonnull)nil); // no-warning
id obj = nil;
foo(obj); // should warning here (implicit cast id to NSString *_Nonnull)

```

Reviewed By: xazax.hun, steakhal

Differential Revision: https://reviews.llvm.org/D154221

clang/lib/StaticAnalyzer/Checkers/NullabilityChecker.cpp

@@ -798,7 +798,7 @@ void NullabilityChecker::checkPreCall(const CallEvent &Call,
     Nullability RequiredNullability =
         getNullabilityAnnotation(Param->getType());
     Nullability ArgExprTypeLevelNullability =
-        getNullabilityAnnotation(ArgExpr->getType());
+        getNullabilityAnnotation(lookThroughImplicitCasts(ArgExpr)->getType());
 
     unsigned ParamIdx = Param->getFunctionScopeIndex() + 1;
 

clang/test/Analysis/nullability-arc.mm

@@ -3,9 +3,6 @@
 // RUN: %clang_analyze_cc1 -w -analyzer-checker=core,nullability\
 // RUN:                       -analyzer-output=text -verify %s -fobjc-arc
 
-#if !__has_feature(objc_arc)
-// expected-no-diagnostics
-#endif
 
 
 #define nil ((id)0)
@@ -24,16 +21,10 @@ @implementation Derived
 - (void)foo:(Param *)param {
   // FIXME: Why do we not emit the warning under ARC?
   [super foo:param];
-#if __has_feature(objc_arc)
-  // expected-warning@-2{{nil passed to a callee that requires a non-null 1st parameter}}
-  // expected-note@-3   {{nil passed to a callee that requires a non-null 1st parameter}}
-#endif
 
   [self foo:nil];
-#if __has_feature(objc_arc)
-  // expected-note@-2{{Calling 'foo:'}}
-  // expected-note@-3{{Passing nil object reference via 1st parameter 'param'}}
-#endif
+  // expected-warning@-1{{nil passed to a callee that requires a non-null 1st parameter}}
+  // expected-note@-2   {{nil passed to a callee that requires a non-null 1st parameter}}
 }
 @end
 

clang/test/Analysis/nullability.mm

@@ -69,6 +69,7 @@ - (void)takesUnspecified:(int *)p;
 void takesNonnull(Dummy *_Nonnull);
 void takesUnspecified(Dummy *);
 void takesNonnullBlock(void (^ _Nonnull)(void));
+void takesNonnullObject(NSObject *_Nonnull);
 
 Dummy *_Nullable returnsNullable();
 Dummy *_Nonnull returnsNonnull();
@@ -275,6 +276,17 @@ void testObjCPropertyReadNullability() {
   return (Dummy * _Nonnull)0;         // no-warning
 }
 
+void testImplicitCastNilToNonnull() {
+  id obj = nil;
+  takesNonnullObject(obj); // expected-warning {{nil passed to a callee that requires a non-null 1st parameter}}
+}
+
+void testImplicitCastNullableArgToNonnull(TestObject *_Nullable obj) {
+  if (!obj) {
+    takesNonnullObject(obj); // expected-warning {{nil passed to a callee that requires a non-null 1st parameter}}
+  }
+}
+
 void testIndirectCastNilToNonnullAndPass() {
   Dummy *p = (Dummy * _Nonnull)0;
   // FIXME: Ideally the cast above would suppress this warning.