This isn't really a new invariant; it effectively already existed due to
existing DataLayout queries. But this makes it explicit.
This is technically not backward-compatible with the existing bitcode
reader, but it's backward-compatible with the output of the bitcode
writer, which is what matters in practice.
No testcase because I don't know a good way to write one: there are no
existing tools that can generate a bitcode file that would trigger the
error.
Split off from D78403.
Differential Revision: https://reviews.llvm.org/D79900
44ecaabc07