To avoid errors where a non-exception .o is on the stack between a throw

To avoid errors where a non-exception .o is on the stack between a throw
and a handler, which would produce errors like:
  terminate called after throwing an instance of 'std::string'
we must comment out setting -fno-exceptions until PR797 is completely
fixed. Once libraries like lib/System and lib/Support are exception free,
we can turn it back on.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@29768 91177308-0d34-0410-b5e6-96231b3b80d8