Partial apply for coroutines

The patch adds lowering of partial_apply instructions for coroutines.

This pattern seems to trigger a lot of type mismatch errors in IRGen, because
coroutine functions are not substituted in the same way as regular functions
(see the patch 07f03bd "Use pattern substitutions to consistently abstract
yields" for more details). The odd type conversions in the patch are related to
this issue, and these should be checked carefully. Perhaps it is better to
enable substitutions for coroutine functions instead (at least for some cases).

Other than that, lowering of partial_apply for coroutines is straightforward: we
generate another coroutine that captures arguments passed to the partial_apply
instructions. It calls the original coroutine for yields (first return) and
yields the resulting values. Then it calls the original function's continuation
for return or unwind, and forwards them to the caller as well.

After IRGen, LLVM's Coroutine pass transforms the generated coroutine (along with
all other coroutines) and eliminates llvm.coro.* intrinsics. LIT tests check
LLVM IR after this transformation.

Author: asavonic

lib/IRGen/GenCall.cpp

@@ -5860,13 +5860,13 @@ void irgen::emitYieldOnceCoroutineResult(IRGenFunction &IGF, Explosion &result,
       // Capture results via result token
       resultToken =
         Builder.CreateIntrinsicCall(llvm::Intrinsic::coro_end_results, coroResults);
-
-        Builder.CreateIntrinsicCall(llvm::Intrinsic::coro_end,
-                              {handle,
-                               /*is unwind*/ Builder.getFalse(),
-                               resultToken});
-        Builder.CreateUnreachable();
     }
+
+    Builder.CreateIntrinsicCall(llvm::Intrinsic::coro_end,
+                                {handle,
+                                 /*is unwind*/ Builder.getFalse(),
+                                 resultToken});
+    Builder.CreateUnreachable();
   } else {
     if (coroResults.empty()) {
       // No results, we do not need to change anything around existing coro.end

lib/IRGen/GenDecl.cpp

@@ -6257,6 +6257,7 @@ IRGenModule::getAddrOfContinuationPrototype(CanSILFunctionType fnType) {
   llvm::Function *&entry = GlobalFuncs[entity];
   if (entry) return entry;
 
+  GenericContextScope scope(*this, fnType->getInvocationGenericSignature());
   auto signature = Signature::forCoroutineContinuation(*this, fnType);
   LinkInfo link = LinkInfo::get(*this, entity, NotForDefinition);
   entry = createFunction(*this, link, signature);