[BitcodeReader] datalayout must be specified before it is queried.

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