AsmWriter: Cleanup debug info fields with MDFieldPrinter, NFC

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).

llvm-svn: 233337