[llvm-dwarfdump] - Teach verifier to report broken DWARF expressions.

[llvm-dwarfdump] - Teach verifier to report broken DWARF expressions.

Patch improves next things:

* Fixes assert/crash in getOpDesc when giving it a invalid expression op code.
* DWARFExpression::print() called DWARFExpression::Operation::getEndOffset() which
  returned and used uninitialized field EndOffset. Patch fixes that.
* Teaches verifier to verify DW_AT_location and error out on broken expressions.

Differential revision: https://reviews.llvm.org/D39294

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316756 91177308-0d34-0410-b5e6-96231b3b80d8