Serialization: Intro the UNWRAP macro to simplify bubbling up errors

Non-error resilient call sites like this:

  DeclContext *DC = MF.getDeclContext(contextID);

Can be replaced with this error tolerant alternative:

  DeclContext *DC;
  UNWRAP(MF.getDeclContextChecked(contextID), DC);

Author: xymus

lib/Serialization/Deserialization.cpp

@@ -52,6 +52,16 @@
 
 #define DEBUG_TYPE "Serialization"
 
+// Unwrap an Expected<> variable following the typical deserialization pattern:
+// - On a value, assign it to Output.
+// - On an error, return it to bubble it up to the caller.
+#define UNWRAP(Input, Output) { \
+  auto ValueOrError = Input; \
+  if (!ValueOrError) \
+      return ValueOrError.takeError(); \
+  Output = ValueOrError.get(); \
+}
+
 STATISTIC(NumDeclsLoaded, "# of decls deserialized");
 STATISTIC(NumMemberListsLoaded,
           "# of nominals/extensions whose members were loaded");