Report error for non-zero data in .bss

User may initialize a var with non-zero value and specify .bss section.
E.g. : int a __attribute__((section(".bss"))) = 2;

This patch converts an assertion to error report for better user
experience.

Differential Revision: http://reviews.llvm.org/D4199

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211455 91177308-0d34-0410-b5e6-96231b3b80d8