[llvm-objcopy] - Calculate the string table section sizes correctly.

[llvm-objcopy] - Calculate the string table section sizes correctly.

This fixes the https://bugs.llvm.org/show_bug.cgi?id=40980.

Previously if string optimization occurred as a result of
StringTableBuilder's finalize() method, the size wasn't updated.

This hopefully also makes the interaction between sections during finalization
processes a bit more clear.

Differential revision: https://reviews.llvm.org/D59488

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@356371 91177308-0d34-0410-b5e6-96231b3b80d8