[codeview] Fix assertion failure introduced in r295354 refactoring

CodeViewDebug sets Asm to nullptr to disable debug info generation.  You
can get a .ll file like no-cus.ll from 'clang -gcodeview -g0', which
happens in the ubsan test suite.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@302923 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed