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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@233562 91177308-0d34-0410-b5e6-96231b3b80d8

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