Add field Unit to DISubprogram

[langston-barrett]

In 2016, several parameters to DICompileUnit were shifted back by one index (Why do that, rather than add it on to the end, in the backwards-compatible way, like they do for so many other new fields? 🤔).

That commit also makes the dicuSubprograms field unused, but that's a non-breaking change.

Unit

• Record index: 15
• Type: Maybe PValMd
• Default: Nothing
• Git commit: llvm-mirror/llvm@4eeaa0d#diff-7f530e364c7ddf1ea5bfa80d03d88c6dR1388
• LLVM review: https://reviews.llvm.org/D19034