Fix a crash on invalid found by fuzzing. (#3404)

When a `namespace` keyword has no `;` following it, we recover by
building a parse tree `Namespace` node from the `Namespace` token (as
there isn't a `;` token). Allow this correspondence on errors.

Also teach the diagnostics in this case to avoid the end-of-file token
as that's almost always going to be a less meaningful location. Instead,
we can point at the introducer which should at least be in the code that
led to the error.

toolchain/parse/handle_decl_name_and_params.cpp

@@ -27,7 +27,13 @@ static auto HandleDeclNameAndParams(Context& context, State after_name)
     CARBON_DIAGNOSTIC(ExpectedDeclName, Error,
                       "`{0}` introducer should be followed by a name.",
                       Lex::TokenKind);
-    context.emitter().Emit(*context.position(), ExpectedDeclName,
+    Lex::Token location = *context.position();
+    if (context.tokens().GetKind(location) == Lex::TokenKind::EndOfFile) {
+      // The end of file is often an especially unhelpful location. If that's
+      // the best we can do here, back up the location to the introducer itself.
+      location = state.token;
+    }
+    context.emitter().Emit(location, ExpectedDeclName,
                            context.tokens().GetKind(state.token));
     context.ReturnErrorOnState();
     context.AddLeafNode(NodeKind::InvalidParse, *context.position(),