[DWARF] Warn on and ignore ".file 0" for DWARF v4 and earlier.

This had been messing with the directory table for prior versions, and
also could induce a crash when generating asm output.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@335254 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed