[MC][ELF] compute entity size for explicit sections

Summary:
Global variables might declare themselves to be in explicit sections.
Calculate the entity size always to prevent assembler warnings
"entity size for SHF_MERGE not specified" when sections are to be
marked merge-able.

Fixes PR31828.

Reviewers: rnk, echristo

Reviewed By: rnk

Subscribers: llvm-commits, pirama, srhines

Differential Revision: https://reviews.llvm.org/D53056

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@344197 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed