Fix segfault in hasDeclContext for nodes that have no decl context.

[sbenza]

Summary

Some declarations do not have a declaration context, like TranslationUnitDecl.
Fix hasDeclContext() to not segfault on these nodes.

[klimek]

LG. You don't need pre-submit reviews for obvious crasher fixes, btw :)

[sbenza]

Closed by commit rL220719 (authored by @sbenza).

include/clang/ASTMatchers/ASTMatchers.h

 ///       class D {};
 ///     }
 ///   }
 /// \code
 ///
 /// \c recordDecl(hasDeclContext(namedDecl(hasName("M")))) matches the
 /// declaration of \c class \c D.
 AST_MATCHER_P(Decl, hasDeclContext, internal::Matcher<Decl>, InnerMatcher) {
-  return InnerMatcher.matches(*Decl::castFromDeclContext(Node.getDeclContext()),
-                              Finder, Builder);
+  const DeclContext *DC = Node.getDeclContext();
+  if (!DC) return false;
+  return InnerMatcher.matches(*Decl::castFromDeclContext(DC), Finder, Builder);
 }
 
 /// \brief Matches nested name specifiers.
 ///
 /// Given
 /// \code
 ///   namespace ns {
 ///     struct A { static void f(); };

unittests/ASTMatchers/ASTMatchersTest.cpp

TEST(DeclarationMatcher, hasDeclContext) {
 
   EXPECT_TRUE(matches("namespace {"
                       "  namespace M {"
                       "    class D {};"
                       "  }"
                       "}",
                       recordDecl(hasDeclContext(namespaceDecl(
                           hasName("M"), hasDeclContext(namespaceDecl()))))));
+
+  EXPECT_TRUE(matches("class D{};", decl(hasDeclContext(decl()))));
 }
 
 TEST(DeclarationMatcher, LinkageSpecification) {
   EXPECT_TRUE(matches("extern \"C\" { void foo() {}; }", linkageSpecDecl()));
   EXPECT_TRUE(notMatches("void foo() {};", linkageSpecDecl()));
 }
 
 TEST(ClassTemplate, DoesNotMatchClass) {