IRGen: do not mark AFP as const

We were not marking the async function pointer as constant but adding it
to the `.rdata` section which the Windows linker helped identify as a
problem: we were adding data with "rw" to a section that is expected to
be "rd".  Marking the data as constant results in error propagation
breaking (presumably due to failure to materialize parameters properly
as bindiff'ing the Concurrency library indicates that the functions are
nearly identical with some argument loads elided).  This repairs the
emission of read/write data into the readonly section.

Author: compnerd

lib/IRGen/GenThunk.cpp

@@ -386,7 +386,6 @@ llvm::Constant *IRGenModule::defineAsyncFunctionPointer(LinkEntity entity,
   auto asyncEntity = LinkEntity::forAsyncFunctionPointer(entity);
   auto *var = cast<llvm::GlobalVariable>(
       getAddrOfLLVMVariable(asyncEntity, init, DebugTypeInfo()));
-  setTrueConstGlobal(var);
   return var;
 }