[dwarfdump] Verify that unit type matches root DIE

This patch adds two new verifiers:

  - It checks that the root DIE of a CU is actually a valid unit DIE.
    (based on its tag)
  - For DWARF5 which contains a unit type int he CU header, it checks that
    this matches the type of the unit DIE.

Differential revision: https://reviews.llvm.org/D38453

llvm-svn: 315121