Sema: Always mark initializers of lazy vars as subsumed.
Previously, the initializer expressions of lazy vars would only be marked as
subsumed when the getter body for the var was synthesized. This didn't work
with `-experimental-lazy-typechecking` since accessor synthesis was not
guaranteed to happen. Consequently, SILGen would emit the initializer even
though it was already subsumed and then assert/crash since the init had also
not been checked and contextualized. Now lazy var inits are marked subsumed in
the request creating storage.
Resolves rdar://118421753
