[ELF] Disable ICF for synthetic sections

The Data member of synthetic section's is not valid and empty. The Data
member is required to be valid by ICF as it is used by ICF to determine
the equality of section contents. Therefore, exclude synthetic sections
from ICF.

Fixes bug PR36910.

Differential Revision: https://reviews.llvm.org/D44923

llvm-svn: 328624