AsmWriter: Cleanup debug info fields with MDFieldPrinter, NFC
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).

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@233337 91177308-0d34-0410-b5e6-96231b3b80d8

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