Fix dangling pointer in isDerivedFrom.

[sbenza]

Summary

Replace usage of StringRef with std::string in AST_MATCHER* generated
matchers to make sure they keep their own copy of the string.
The value could be a temporary and it causes the pointer to be dangling
by the time the matcher is executed.

[klimek]

FYI: Usually when you have simple fixes like this that don't need pre-commit review, you don't need to send a patch with arc. You just land them.

include/clang/ASTMatchers/ASTMatchers.h

 ///   class Bar : public Foo {};  // derived from a type that X is a typedef of
 /// \endcode
 AST_MATCHER_P(CXXRecordDecl, isDerivedFrom,
               internal::Matcher<NamedDecl>, Base) {
   return Finder->classIsDerivedFrom(&Node, Base, Builder);
 }
 
 /// \brief Overloaded method as shortcut for \c isDerivedFrom(hasName(...)).
-AST_MATCHER_P_OVERLOAD(CXXRecordDecl, isDerivedFrom, StringRef, BaseName, 1) {
+AST_MATCHER_P_OVERLOAD(CXXRecordDecl, isDerivedFrom, std::string, BaseName, 1) {
   assert(!BaseName.empty());
   return isDerivedFrom(hasName(BaseName)).matches(Node, Finder, Builder);
 }
 
 /// \brief Similar to \c isDerivedFrom(), but also matches classes that directly
 /// match \c Base.
 AST_MATCHER_P_OVERLOAD(CXXRecordDecl, isSameOrDerivedFrom,
                        internal::Matcher<NamedDecl>, Base, 0) {
   return Matcher<CXXRecordDecl>(anyOf(Base, isDerivedFrom(Base)))
       .matches(Node, Finder, Builder);
 }
 
 /// \brief Overloaded method as shortcut for
 /// \c isSameOrDerivedFrom(hasName(...)).
-AST_MATCHER_P_OVERLOAD(CXXRecordDecl, isSameOrDerivedFrom, StringRef, BaseName,
-                       1) {
+AST_MATCHER_P_OVERLOAD(CXXRecordDecl, isSameOrDerivedFrom, std::string,
+                       BaseName, 1) {
   assert(!BaseName.empty());
   return isSameOrDerivedFrom(hasName(BaseName)).matches(Node, Finder, Builder);
 }
 
 /// \brief Matches the first method of a class or struct that satisfies \c
 /// InnerMatcher.
 ///
 /// Given: