Add an addAbsolute static function to Writer.cpp

Summary:
SymbolTable::addAbsolute() was removed in rL344305.
To me this is more readable than the lambda named `Add` and in our
out-of-tree CHERI target we use addAbsolute() in another function.

Reviewers: ruiu, espindola

Reviewed By: ruiu

Subscribers: kristina, emaste, llvm-commits

Differential Revision: https://reviews.llvm.org/D53393

llvm-svn: 344842