Move all the `MDNode` field helper methods into a new class,
`MDFieldPrinter`, and add helpers for integers, bools, and `DW_*`
symbolic constants. This reduces a ton of code duplication, and makes
it more mechanical to update `AsmWriter` to print broken code in the
context of stricter accessors (like in r233322).
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233337 91177308-0d34-0410-b5e6-96231b3b80d8