[PATCH] Support: Add dwarf::getVirtuality()

[Duncan P. N. Exon Smith]

DW_VIRTUALITY_* (the last DW_* thing that my upcoming assembly patches
use) is a bit different from DW_TAG, DW_LANG and DW_ATE.

There are only three valid values:

  - 0x0 (none),
  - 0x1 (virtual), and
  - 0x2 (pure_virtual),

and there isn't a `DW_VIRTUALITY_lo_user` or `DW_VIRTUALITY_hi_user`.

The assembly code needs to know the valid numeric range, so I've added
a `DW_VIRTUALITY_max` that also points at `0x2`.  I've also added a
`DW_VIRTUALITY_invalid` to `LLVMConstants` to return as an error code
from `getVirtuality()`.

I was about to commit without asking, but since it's a little different
I thought I'd check.  Does this make sense to others?

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