[ubsan] Don't enable debug info in all tests

Add a lit substitution (I chose %gmlt) so that only stack trace tests
get debug info.

We need a lit substition so that this expands to -gline-tables-only
-gcodeview on Windows. I think in the future we should reconsider the
need for -gcodeview from the GCC driver, but for now, this is necessary.

llvm-svn: 303083