[coroutines] Add DependentCoawaitExpr and fix re-building CoroutineBodyStmt.

Summary:
The changes contained in this patch are:

1. Defines a new AST node `CoawaitDependentExpr` for representing co_await expressions while the promise type is still dependent.
2. Correctly detect and transform the 'co_await' operand to  `p.await_transform(<expr>)`  when possible.
3. Change the initial/final suspend points to build during the initial parse, so they have the correct operator co_await lookup results.
4.  Fix transformation of the CoroutineBodyStmt so that it doesn't re-build the final/initial suspends.


@rsmith: This change is a little big, but it's not trivial for me to split it up. Please let me know if you would prefer this submitted as multiple patches.



Reviewers: rsmith, GorNishanov

Reviewed By: rsmith

Subscribers: ABataev, rsmith, mehdi_amini, cfe-commits

Differential Revision: https://reviews.llvm.org/D26057

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@297093 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 5 insertions, 0 deletions

diff --git a/lib/Serialization/ASTWriterStmt.cpp b/lib/Serialization/ASTWriterStmt.cpp
index 3543016a3b..bade553425 100644
--- a/lib/Serialization/ASTWriterStmt.cpp
+++ b/lib/Serialization/ASTWriterStmt.cpp
@@ -315,6 +315,11 @@ void ASTStmtWriter::VisitCoawaitExpr(CoawaitExpr *S) {
   llvm_unreachable("unimplemented");
 }
 
+void ASTStmtWriter::VisitDependentCoawaitExpr(DependentCoawaitExpr *S) {
+  // FIXME: Implement coroutine serialization.
+  llvm_unreachable("unimplemented");
+}
+
 void ASTStmtWriter::VisitCoyieldExpr(CoyieldExpr *S) {
   // FIXME: Implement coroutine serialization.
   llvm_unreachable("unimplemented");