[cfe] Refactor FunctionNode.futureValueType into emitted value type

Previously we would encode the type of the value returned in `async`
functions as the field `futureValueType` on `FunctionNode`. For all
other kinds of functions, such as `sync`, `sync*`, and `async*`, that
field would be null. This CL renames `futureValueType` into
`emittedVAlueType`, and for functions of kinds `async`, `sync*`, and
`async*` that is expected to be the type of values emitted via
`return` or `yield` statements. For `sync` functions that field is
supposed to contain `null`.

In response to https://github.com/dart-lang/sdk/issues/54159

TEST=existing

Change-Id: I1efdbcc4e75d150f5618c7ca50cfe49a0e54fce6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/341662
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Ömer Ağacan <omersa@google.com>
Commit-Queue: Chloe Stefantsova <cstefantsova@google.com>
Reviewed-by: Mayank Patke <fishythefish@google.com>

pkg/kernel/lib/binary/ast_from_binary.dart

@@ -2055,7 +2055,7 @@ class BinaryBuilder {
         returnType: returnType,
         asyncMarker: asyncMarker,
         dartAsyncMarker: dartAsyncMarker,
-        futureValueType: futureValueType)
+        emittedValueType: futureValueType)
       ..fileOffset = offset
       ..fileEndOffset = endOffset
       ..redirectingFactoryTarget = redirectingFactoryTarget;