[clang-repl] Keep the first llvm::Module empty to avoid invalid memory access. (#89031)

Clang's CodeGen is designed to work with a single llvm::Module. In many
cases
for convenience various CodeGen parts have a reference to the
llvm::Module
(TheModule or Module) which does not change when a new module is pushed.
However, the execution engine wants to take ownership of the module
which does
not map well to CodeGen's design. To work this around we clone the
module and
pass it down.

With some effort it is possible to teach CodeGen to ask the
CodeGenModule for
its current module and that would have an overall positive impact on
CodeGen
improving the encapsulation of various parts but that's not resilient to
future
regression.

This patch takes a more conservative approach and keeps the first
llvm::Module
empty intentionally and does not pass it to the Jit. That's also not
bullet
proof because we have to guarantee that CodeGen does not write on the
blueprint. However, we have inserted some assertions to catch accidental
additions to that canary module.

This change will fixes a long-standing invalid memory access reported by
valgrind when we enable the TBAA optimization passes. It also unblock
progress
on https://github.com/llvm/llvm-project/pull/84758.