Implement 'extern library' support for functions. (#4220)

Support for types (particularly classes) is left as a TODO.

There's also an issue I'm observing with a "define in impl" test, but
this is probably an issue with resolving the prior declaration which is
imported indirectly. The PR was already feeling big, so I'm choosing to
cut here.

Note, this does not implement the rule "The owning library's API file
must import the `extern` declaration, and must also contain a
declaration."

toolchain/check/handle_struct.cpp

@@ -29,7 +29,7 @@ auto HandleParseNode(Context& context, Parse::StructLiteralStartId node_id)
 auto HandleParseNode(Context& context,
                      Parse::StructFieldDesignatorId /*node_id*/) -> bool {
   // This leaves the designated name on top because the `.` isn't interesting.
-  CARBON_CHECK(context.node_stack().PeekIsName());
+  CARBON_CHECK(context.node_stack().PeekIs<SemIR::NameId>());
   return true;
 }