Add a missing enumerator to this switch. Currently its in the

Add a missing enumerator to this switch. Currently its in the
assert-path code, as previously we would have fallen off the end of the
function, but please review and let me know if this should go somewhere
else.

This fixes a Clang warning:
lib/MC/MCMachOStreamer.cpp:201:11: error: enumeration value 'MCSA_IndirectSymbol' not handled in switch [-Werror,-Wswitch-enum]
  switch (Attribute) {
          ^
1 error generated.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@135976 91177308-0d34-0410-b5e6-96231b3b80d8