Force generic parameter types declared in module context to belong to

the current SIL module, unless the AST sees them first.

This turns out to be important when we deserialize generic
reabstraction thunks, which have shared linkage so get serialized
again in the current SIL module.

There are some fundamental inconsistencies in the way parameter type
decls are handled, but we need a quick workaround to unblock progress
on the stdlib.

Fixes
<rdar://problem/16807985> Building Foundation overlay crashes in stdlib deserialization in r17377

Swift SVN r18173

lib/Serialization/Serialization.cpp

@@ -1136,6 +1136,8 @@ void Serializer::writeCrossReference(const Decl *D) {
   writeCrossReference(D->getDeclContext());
 
   if (auto genericParam = dyn_cast<GenericTypeParamDecl>(D)) {
+    assert(!D->getDeclContext()->isModuleScopeContext() &&
+           "Cannot cross reference a generic type decl at module scope.");
     abbrCode = DeclTypeAbbrCodes[XRefGenericParamPathPieceLayout::Code];
     XRefGenericParamPathPieceLayout::emitRecord(Out, ScratchRecord, abbrCode,
                                                 genericParam->getIndex());