[serialization] Handle cross-module references to values in extensions.

Previously, cross-references used a simple access path to refer to values
in other modules, but extensions have no name. They also accidentally
picked up values in extensions anyway, because lookupDirect includes
members in extensions. Now, we filter out values that don't come from
the referenced module, which may not be the same module the base type
comes from.

Swift SVN r6301

lib/Serialization/ModuleFormat.h

@@ -72,9 +72,10 @@ using AbstractCCField = BCFixed<2>;
 // VERSION_MAJOR.
 enum XRefKind : uint8_t {
   SwiftValue = 0,
+  SwiftExtensionValue,
   SwiftOperator
 };
-using XRefKindField = BCFixed<1>;
+using XRefKindField = BCFixed<2>;
 
 // These IDs must \em not be renumbered or reordered without incrementing
 // VERSION_MAJOR.
@@ -637,7 +638,9 @@ namespace decls_block {
     XREF,
     XRefKindField, // reference kind
     TypeIDField,   // type if value, operator kind if operator
-    BCArray<IdentifierIDField> // access path
+    BCArray<IdentifierIDField> // extension module name (if extension value)
+                               // base module name
+                               // access path
   >;
 }