Verifier: Loosen r233559 check for 'function:' field in MDSubprogram

Stop worrying about what the `function:` field is in `MDSubprogram`,
since it could be a bitcast [1].  Just check its type and leave it at
that.

[1]: http://lab.llvm.org:8080/green/job/clang-stage2-configure-Rlto_build/3540/

llvm-svn: 233562