dwarfdump: Add support for dumping the table contents of DWP indexes

This is a recommit of 252842 which was reverted in 252859. The issue was
using %s format specifier for a StringRef - used Format's
left_justify(StringRef, int) instead.

It'd be nice to have __attribute__((format(..))) on llvm::format, but
apparently it's only implemented for c-style variadics, not C++ variadic
templates. Perhaps we could fix that & conditionalize the attribute on
such...

llvm-svn: 253065