DebugInfo: Introduce the notion of "form classes"

DebugInfo: Introduce the notion of "form classes"

Summary:
Use DWARF4 table of form classes to fetch attributes from DIE
in a more consistent way. This shouldn't change the functionality and
serves as a refactoring for upcoming change: DW_AT_high_pc has different
semantics depending on its form class.

Reviewers: dblaikie, echristo

Reviewed By: echristo

CC: echristo, llvm-commits
Differential Revision: http://llvm-reviews.chandlerc.com/D1961

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193553 91177308-0d34-0410-b5e6-96231b3b80d8