Support module importing with differing code models

First, module-level code model information is stored as a module flag.
To support merging of modules with different module flag values, we add
a new module flag behavior, `Ignore`, which ignores differences between
values and takes the flag from the first module.

Second, when a module imports a value from another module with a
dfferent code model, the imported value should retain the code model of
its source module. A small value imported into a large module should
remain small in order to prevent disturbing other references to that
small value in its original module. At the same time, a large value
imported into a small module should remain large as otherwise the value
may not be reachable in the small value.

Co-authored-by: Alan Zhao <ayzhao@google.com>