cc1as: Don't crash when CIE is requested and no DWARF version is specified.

This patch changes the default DWARF version for cc1as from invalid 0 to 2,
which should be the lowest common denominator on all platforms.

rdar://problem/24735813

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@266814 91177308-0d34-0410-b5e6-96231b3b80d8