Make the "typechecked function body" request more central and resilient

The "typechecked function body" request was defined to type-check a
function body that is known to be present, and not skipped, and would
assert these conditions, requiring its users to check whether a body
was expected. Often, this means that callers would use `getBody()`
instead, which retrieves the underlying value in whatever form it
happens to be, and assume it has been mutated appropriately.

Make the "typechecked function body" request, triggered by
`getTypecheckedBody()`, more resilient and central. A `NULL` result is
now acceptable, signifying that there is no body. Clients will need to
tolerate NULL results.

* When there is no body but should be one, produce an appropriate
error.
* When there shouldn't be a body but is, produce an appropriate error
* Handle skipping of function bodies here, rather than elsewhere.

Over time, we should move clients off of `getBody` and `hasBody`
entirely, and toward `getTypecheckedBody` or some yet-to-be-introduced
forms like `getBodyAsWritten` for the pre-typechecked body.

lib/Sema/DebuggerTestingTransform.cpp

@@ -88,7 +88,7 @@ public:
     // Whitelist the kinds of decls to transform.
     // TODO: Expand the set of decls visited here.
     if (auto *FD = dyn_cast<AbstractFunctionDecl>(D))
-      return Action::VisitChildrenIf(FD->getBody());
+      return Action::VisitChildrenIf(FD->getTypecheckedBody());
     if (auto *TLCD = dyn_cast<TopLevelCodeDecl>(D))
       return Action::VisitChildrenIf(TLCD->getBody());
     if (isa<NominalTypeDecl>(D))