DWARF: Unify form size handling code

Summary:
This patch replaces the two switches which are deducing the size of
various forms with a single implementation. I have put the new
implementation into BinaryFormat, to avoid introducing dependencies
between the two independent libraries (DebugInfo and CodeGen) that need
this functionality.

Reviewers: aprantl, JDevlieghere, dblaikie

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D44418

llvm-svn: 327486