[modules] Remove redundant import of lexical decls when building a lookup table

for a DeclContext, and fix propagation of exception specifications along
redeclaration chains.

This reverts r232905, r232907, and r232907, which reverted r232793, r232853,
and r232853.

One additional change is present here to resolve issues with LLDB: distinguish
between whether lexical decls missing from the lookup table are local or are
provided by the external AST source, and still look in the external source if
that's where they came from.

llvm-svn: 232928

clang/lib/Sema/SemaExceptionSpec.cpp

@@ -167,13 +167,13 @@ Sema::ResolveExceptionSpec(SourceLocation Loc, const FunctionProtoType *FPT) {
 void
 Sema::UpdateExceptionSpec(FunctionDecl *FD,
                           const FunctionProtoType::ExceptionSpecInfo &ESI) {
-  for (auto *Redecl : FD->redecls())
-    Context.adjustExceptionSpec(cast<FunctionDecl>(Redecl), ESI);
-
   // If we've fully resolved the exception specification, notify listeners.
   if (!isUnresolvedExceptionSpec(ESI.Type))
     if (auto *Listener = getASTMutationListener())
       Listener->ResolvedExceptionSpec(FD);
+
+  for (auto *Redecl : FD->redecls())
+    Context.adjustExceptionSpec(cast<FunctionDecl>(Redecl), ESI);
 }
 
 /// Determine whether a function has an implicitly-generated exception