Delay the emission of SIL global variables that aren't externally
visible until they are actually used. This is the same lazy emission
approach that we use for a number of other entities, such as SIL
functions.
Part of rdar://158363967.
Introduce an experimental feature DeferredCodeGen, that defers the
generation of LLVM IR (and therefore object code) for all entities
within an Embedded Swift module unless they have explicitly requested
to not be emitted into the client (e.g., with
`@_neverEmitIntoClient`).
This feature is meant to generalize and subsume
-emit-empty-object-file, relying on lazy emission of entities rather
than abruptly ending the compilation pipeline before emitting any IR.
Part of rdar://158363967.
When Embedded Swift emits a symbol that was imported from another
module, ensure that the symbol is emitted as a weak definition. This
way, importing the same module (and using its symbol) into several
different modules doesn't cause duplicate-symbol errors at link time.
Rather, the linker will merge the different symbol definitions. This
makes Embedded Swift libraries work without resorting to
`-mergeable-symbols` or `-emit-empty-object-file`.
