libclang: reduce CXString abuse

libclang: reduce CXString abuse

ContainerUSR is not really a CXString, but it should own the underlying memory
buffer.  Thus, it is better to change the type to std::string.  This will not
introduce extra copying overhead, since the workaround that is being removed
was already making a copy.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174647 91177308-0d34-0410-b5e6-96231b3b80d8