[serialization] Add more metadata for traceability (eg LLVM version)

[derek-gerstmann]

It'd be nice to add more metadata to the serialization format to enable tools to inject more specific details on where the serialization came from (eg. to enable caching of the data within a larger toolchain).

In particular, adding the LLVM version (particularly when Halide is built from source and not part of a release) would be super useful!