Fixed a case where the result of std::string's c_str() method was being called...

Fixed a case where the result of std::string's c_str() method was being called on a local variable and returned as a const char * incorrectly. We used to cache the thread names for threads in the current host process, but we shoudn't be caching that as the names can change over time, so now a std::string is returned from Host::GetThreadName().

llvm-svn: 176217