dwarfdump: Delay parsing abbreviations until they're needed

dwarfdump: Delay parsing abbreviations until they're needed

This speeds up dumping specific DIEs by not parsing abbreviations for
units that are not used.

(this is also handy to have in eventually to speed up llvm-symbolizer
for .dwp files, where parsing most of the DWP file can be avoided by
using the index)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@313635 91177308-0d34-0410-b5e6-96231b3b80d8