[llvm] [unittest] Allow getting a C string from the TempDir helper class

The TempDir.path() member function returns a StringRef. We've been
calling the data() method on that StringRef, which does not guarantee
to return a null-terminated string (required by chdir and other POSIX
functions).

Introduce the c_str() method in the TempDir class, which returns the
proper string without the need to create a copy of the path at use site.