[llvm-readobj] Update readobj to re-use parsing code.

[llvm-readobj] Update readobj to re-use parsing code.

llvm-readobj hand rolls some CodeView parsing code for string
tables, so this patch updates it to re-use some of the newly
introduced parsing code in LLVMDebugInfoCodeView.

Differential Revision: https://reviews.llvm.org/D32772

llvm-svn: 302052