llvm-dwarfdump: Add an option to collect debug info quality metrics.

llvm-dwarfdump: Add an option to collect debug info quality metrics.

At the last LLVM dev meeting we had a debug info for optimized code
BoF session. In that session I presented some graphs that showed how
the quality of the debug info produced by LLVM changed over the last
couple of years. This is a cleaned up version of the patch I used to
collect the this data. It is implemented as an extension of
llvm-dwarfdump, adding a new --statistics option. The intended
use-case is to automatically run this on the debug info produced by,
e.g., our bots, to identify eyebrow-raising changes or regressions
introduced by new transformations that we could act on.

In the current form, two kinds of data are being collected:

- The number of variables that have a debug location versus the number
  of variables in total (this takes into account inlined instances of
  the same function, so if a variable is completely missing form only
  one instance it will be found).

- The PC range covered by variable location descriptions versus the PC
  range of all variables' containing lexical scopes.

The output format is versioned and extensible, so I'm looking forward
to both bug fixes and ideas for other data that would be interesting
to track.

Differential Revision: https://reviews.llvm.org/D36627

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315101 91177308-0d34-0410-b5e6-96231b3b80d8