Add a request to lazily parse function bodies.

Rework the lazy function body parsing mechanism to use the
request-evaluator, so that asking for the body of a function will
initiate parsing. Clean up a number of callers to
AbstractFunctionDecl::getBody() that don't actually need the body, so
we don't perform unnecessary parsing.

This change does not delay parsing of function bodies in the general
case; rather, it sets up the infrastructure to always delay parsing of
function bodies.

lib/Sema/SourceLoader.cpp

@@ -152,9 +152,6 @@ ModuleDecl *SourceLoader::loadModule(SourceLoc importLoc,
   assert(done && "Parser returned early?");
   (void)done;
 
-  if (SkipBodies)
-    performDelayedParsing(importMod, persistentState, nullptr);
-
   // FIXME: Support recursive definitions in immediate modes by making type
   // checking even lazier.
   if (SkipBodies)