[TBDGen] @asyncHandlers don't have public async function pointers.

Previously, an async function pointer symbol was being emitted into the
TBD for all AbstractFunctionDecls which returned true for
"hasAsynContext", i.e. both those that were async and those which were
asnycHandlers.  That was not correct because the async function portion
of an asyncHandler is not ABI.  Here, that mistake is fixed by only
emitting this symbol for functions which are annotated async (i.e. those
for which "hasAsync" returns true).

rdar://problem/72329062

Author: nate-chandler

lib/TBDGen/TBDGen.cpp

@@ -721,7 +721,7 @@ void TBDGenVisitor::visitAbstractFunctionDecl(AbstractFunctionDecl *AFD) {
 
   visitDefaultArguments(AFD, AFD->getParameters());
 
-  if (AFD->isAsyncContext()) {
+  if (AFD->hasAsync()) {
     addSymbol(LinkEntity::forAsyncFunctionPointer(AFD));
   }
 }

test/TBD/async-function-pointer.swift

@@ -0,0 +1,7 @@
+// REQUIRES: VENDOR=apple 
+// RUN: %target-swift-frontend -emit-ir %s -enable-experimental-concurrency -validate-tbd-against-ir=all -module-name test | %FileCheck %s
+
+// CHECK: @"$s4test6testityyYFAD" = hidden global %swift.async_func_pointer
+
+@asyncHandler
+public func testit() { }