Verifier: fix bug treating debug info issue as non-debug info issue

Normally when llvm-as sees only debug info errors in LLVM assembly, it simply
drops the debug info and outputs a valid LLVM bitcode and returns 0.

There is a bug in LLVM verifier which incorrectly treats a debug info error
as non-debug info error, which causes llvm-as returns 1 even though llvm-as
can drop the invalid debug info and outputs a valid LLVM bitcode.

This patch fixes that.

Differential Revision: https://reviews.llvm.org/D42391

llvm-svn: 323216