[DWARF v5] Rework of string offsets table reader

Reorganizes the DWARF consumer to derive the string offsets table 
contribution's format from the contribution header instead of 
(incorrectly) from the unit's format.

Reviewers: JDevliegehere, aprantl

Differential Revision: https://reviews.llvm.org/D41146
 

llvm-svn: 321295