Port the strip-invalid-debuginfo logic to the legacy verifier pass, too.

Since r268966 the modern Verifier pass defaults to stripping invalid debug info
in nonasserts builds.  This patch ports this behavior back to the legacy
Verifier pass as well.  The primary motivation is that the clang frontend
accepts bitcode files as input but is still using the legacy pass pipeline.

Background: The problem I'm trying to solve with this sequence of patches is
that historically we've done a really bad job at verifying debug info. We want
to be able to make the verifier stricter without having to worry about breaking
bitcode compatibility with existing producers. For example, we don't necessarily
want IR produced by an older version of clang to be rejected by an LTO link just
because of malformed debug info, and rather provide an option to strip it. Note
that merely outdated (but well-formed) debug info would continue to be
auto-upgraded in this scenario.

http://reviews.llvm.org/D20629
<rdar://problem/26448800>

llvm-svn: 270768