We now compute captures of functions and default arguments
lazily, instead of as a side effect of primary file checking.
Captures of closures are computed as part of the enclosing
context, not lazily, because the type checking of a single
closure body is not lazy.
This fixes a specific issue with the `-experimental-skip-*` flags,
where functions declared after a top-level `guard` statement are
considered to have local captures, but nothing was forcing these
captures to be computed.
Fixes rdar://problem/125981663.
