Implement a safer bitcode upgrade for DISubprogram.

Implement a safer bitcode upgrade for DISubprogram.

The bitcode upgrade I added for DISubprogram in r266446 was based on the
assumption that the CU node for the subprogram was already materialized by the
time the DISubprogram is visited. This assumption may not hold true as future
versions of LLVM may decide to write out bitcode in a different order. This
patch corrects this by introducing a versioning bit next to the distinct flag to
unambiguously differentiate the new from the old record layouts.

Note for people stabilizing LLVM out-of-tree: This patch introduces a bitcode
incompatibility with llvm trunk revisions from r266446 — this commit. (But
D19987 will ensure that it degrades gracefully).

http://reviews.llvm.org/D20004
rdar://problem/26074194

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@268816 91177308-0d34-0410-b5e6-96231b3b80d8