DebugInfo: Move DISubprogram::is*() queries to MDSubprogram

Move body of `DISubprogram::isPrivate()` (etc.) to `MDSubprogram`, and
change the versions in `DISubprogram` to forward there.

This is just like r234275, but for subprograms instead of types.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234282 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed