Change llvm.exp so it no longer ignores some errors when executing dejagnu

tests. This breaks 80 tests in the tree.

The interesting part here is that this no longer ignores syntax errors
in RUN command lines. Some tests have not been working all the time because of
this.

The tricky part is that it now also views any stderr output as an error. This
can be suppressed in tcl 8.5, but let's not add this dependency. Instead, all
testcases should be changed to redirect stderr if they expect stderr output.
This holds in particular for lines like:
  ; RUN: not llvm-as < %s
where an error is expected (but I think I can solve this by modifying the not
script). Also, compilations resulting in warnings will now also fail (so
the warnings should be fixed, disabled or redirected...).

I'll continue with fixing the testcases that are broken now.

llvm-svn: 52172