Revert "SILGen: Simplify SILGenFunction::emitClosure()"

This change has a subtle impact on debug info emission and causes
a regression with a subsequent patch I'm about to commit.

This reverts commit 62d1adb.

Author: slavapestov

lib/SILGen/SILGenFunction.cpp

@@ -513,7 +513,12 @@ void SILGenFunction::emitClosure(AbstractClosureExpr *ace) {
     emitStmt(ce->getBody());
   } else {
     auto *autoclosure = cast<AutoClosureExpr>(ace);
-    emitStmt(autoclosure->getBody());
+    // Closure expressions implicitly return the result of their body
+    // expression.
+    if (B.hasValidInsertionPoint()) {
+      emitReturnExpr(ImplicitReturnLocation(ace),
+                     autoclosure->getSingleExpressionBody());
+    }
   }
   emitEpilog(ace);
 }

lib/SILGen/SILGenStmt.cpp

@@ -482,6 +482,7 @@ void SILGenFunction::emitReturnExpr(SILLocation branchLoc,
     RValue RV = emitRValue(ret).ensurePlusOne(*this, CleanupLocation(ret));
     std::move(RV).forwardAll(*this, directResults);
   }
+
   Cleanups.emitBranchAndCleanups(ReturnDest, branchLoc, directResults);
 }