[analyzer] LocalizationChecker: Fix a crash on synthesized accessor stubs.

The checker was trying to analyze the body of every method in Objective-C
@implementation clause but the sythesized accessor stubs that were introduced
into it by 2073dd2 have no bodies.

(cherry picked from commit b01012b)

Author: haoNoQ

clang/lib/StaticAnalyzer/Checkers/LocalizationChecker.cpp

@@ -1078,7 +1078,10 @@ void EmptyLocalizationContextChecker::checkASTDecl(
     AnalysisDeclContext *DCtx = Mgr.getAnalysisDeclContext(M);
 
     const Stmt *Body = M->getBody();
-    assert(Body);
+    if (!Body) {
+      assert(M->isSynthesizedAccessorStub());
+      continue;
+    }
 
     MethodCrawler MC(M->getCanonicalDecl(), BR, this, Mgr, DCtx);
     MC.VisitStmt(Body);

clang/test/Analysis/localization-aggressive.m

@@ -293,3 +293,11 @@ - (void)testTakesLocalizedString {
   takesLocalizedString(@"not localized"); // expected-warning {{User-facing text should use localized string macro}}
 }
 @end
+
+@interface SynthesizedAccessors : NSObject
+@property (assign) NSObject *obj;
+@end
+
+@implementation SynthesizedAccessors
+// no-crash
+@end