[Typed throws] Record thrown error types and conversions in the AST

For any operation that can throw an error, such as calls, property
accesses, and non-exhaustive do..catch statements, record the thrown
error type along with the conversion from that thrown error to the
error type expected in context, as appropriate. This will prevent
later stages from having to re-compute the conversion sequences.

lib/Sema/DebuggerTestingTransform.cpp

@@ -253,7 +253,7 @@ private:
 
     auto *POArgList = ArgumentList::forImplicitUnlabeled(Ctx, {DstRef});
     auto *POCall = CallExpr::createImplicit(Ctx, PODeclRef, POArgList);
-    POCall->setThrows(false);
+    POCall->setThrows(nullptr);
 
     // Create the call to checkExpect.
     UnresolvedDeclRefExpr *CheckExpectDRE = new (Ctx)
@@ -263,7 +263,7 @@ private:
         ArgumentList::forImplicitUnlabeled(Ctx, {Varname, POCall});
     auto *CheckExpectExpr =
         CallExpr::createImplicit(Ctx, CheckExpectDRE, CheckArgList);
-    CheckExpectExpr->setThrows(false);
+    CheckExpectExpr->setThrows(nullptr);
 
     // Create the closure.
     auto *Params = ParameterList::createEmpty(Ctx);
@@ -283,7 +283,7 @@ private:
 
     // Call the closure.
     auto *ClosureCall = CallExpr::createImplicitEmpty(Ctx, Closure);
-    ClosureCall->setThrows(false);
+    ClosureCall->setThrows(nullptr);
 
     // TODO: typeCheckExpression() seems to assign types to everything here,
     // but may not be sufficient in some cases.