Fix several issues with the test-release.sh script

* Use the default install prefix (/usr/local) and use DESTDIR instead to
  set a temporary install location for tarballing. This is the correct
  way to package binary releases (otherwise the temporary install path
  ends up in files in the binary release).
* Remove ``-disable-clang`` option. It did not work correctly
  (tarballing assumed phase 3 was run) and when doing a release
  we should always be doing a three-phased build and test.

Note: Technically we should only be using DESTDIR for the third phase
and use --prefix for the first and second phase because we run the built
clang from phase 1 and 2 (and in general an application's behaviour
may depend on the install prefix). However in the case of clang it
seems to not care what the install prefix was so to simplify the script
we use DESTDIR for all three stages.

llvm-svn: 242187