swift234

[serialization] Add a metaprogramming system for bitcode record layout.

Alternately, "add a system of ridiculous overkill for bitcode record layout",
in the spirit of DRY over YAGNI. It /is/ much more concise than the LLVM way.

Use this to emit a sketch of Clang's PCH control block, although nothing's in
there yet besides "version 1.0" and the LLVM revision number; the latter is
more "proof of concept" than actually useful. We should figure out a good
way to identify trunk Swift compilers; this shouldn't be difficult.

Swift SVN r5107