[DWARF] Modification of code for the verification of .debug_info section.

Summary:
This patch modifies the handleDebugInfo() function so that we verify the contents of each unit
in the .debug_info section only if its header has been successfully verified.

This change will allow for more/different verification checks depending on the type of the unit since from
dwarf5, the .debug_info section may consist of different types of units.

Subscribers: aprantl

Differential Revision: https://reviews.llvm.org/D35521

llvm-svn: 308245