Fix dangling pointer in isDerivedFrom.

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.

Reviewers: klimek

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D6843

llvm-svn: 225180