[Coroutines] Convert coroutine.presplit to enum attr

This is required by @nikic in https://reviews.llvm.org/D127383 to
decrease the cost to check whether a function is a coroutine and this
fixes a FIXME too.

Reviewed By: rjmccall, ezhulenev

Differential Revision: https://reviews.llvm.org/D127471

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -1634,6 +1634,8 @@ static Attribute::AttrKind getAttrFromCode(uint64_t Code) {
     return Attribute::MustProgress;
   case bitc::ATTR_KIND_HOT:
     return Attribute::Hot;
+  case bitc::ATTR_KIND_PRESPLIT_COROUTINE:
+    return Attribute::PresplitCoroutine;
   }
 }