[CodeGen] Print external symbols as $symbol in both MIR and debug output

Work towards the unification of MIR and debug output by printing
`$symbol` instead of `<es:symbol>`.

Only debug syntax is affected.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@320681 91177308-0d34-0410-b5e6-96231b3b80d8
8 files changed