Sema: Convert TypeChecker::computeCaptures() into two requests

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.

lib/AST/Module.cpp

@@ -3468,6 +3468,7 @@ void SourceFile::typeCheckDelayedFunctions() {
     auto *AFD = DelayedFunctions[i];
     assert(!AFD->getDeclContext()->isLocalContext());
     AFD->getTypecheckedBody();
+    (void) AFD->getCaptureInfo();
   }
 
   DelayedFunctions.clear();