Migrate parts of rustc_expand to session diagnostics

This migrates everything but the `mbe` and `proc_macro` modules. It also
contains a few cleanups and drive-by/accidental diagnostic improvements
which can be seen in the diff for the UI tests.

compiler/rustc_error_messages/locales/en-US/expand.ftl

@@ -20,3 +20,110 @@ expand_var_still_repeating =
     variable '{$ident}' is still repeating at this depth
 
 expand_meta_var_dif_seq_matchers = {$msg}
+
+expand_macro_const_stability =
+    macros cannot have const stability attributes
+    .label = invalid const stability attribute
+    .label2 = const stability attribute affects this macro
+
+expand_macro_body_stability =
+    macros cannot have body stability attributes
+    .label = invalid body stability attribute
+    .label2 = body stability attribute affects this macro
+
+expand_resolve_relative_path =
+    cannot resolve relative path in non-file source `{$path}`
+
+expand_attr_no_arguments =
+    attribute must have either one or two arguments
+
+expand_not_a_meta_item =
+    not a meta item
+
+expand_only_one_word =
+    must only be one word
+
+expand_cannot_be_name_of_macro =
+    `{$trait_ident}` cannot be a name of {$macro_type} macro
+
+expand_arg_not_attributes =
+    second argument must be `attributes`
+
+expand_attributes_wrong_form =
+    attribute must be of form: `attributes(foo, bar)`
+
+expand_attribute_meta_item =
+    attribute must be a meta item, not a literal
+
+expand_attribute_single_word =
+    attribute must only be a single word
+
+expand_helper_attribute_name_invalid =
+    `{$name}` cannot be a name of derive helper attribute
+
+expand_expected_comma_in_list =
+    expected token: `,`
+
+expand_only_one_argument =
+    {$name} takes 1 argument
+
+expand_takes_no_arguments =
+    {$name} takes no arguments
+
+expand_feature_included_in_edition =
+    the feature `{$feature}` is included in the Rust {$edition} edition
+
+expand_feature_removed =
+    feature has been removed
+    .label = feature has been removed
+    .reason = {$reason}
+
+expand_feature_not_allowed =
+    the feature `{$name}` is not in the list of allowed features
+
+expand_recursion_limit_reached =
+    recursion limit reached while expanding `{$descr}`
+    .help = consider increasing the recursion limit by adding a `#![recursion_limit = "{$suggested_limit}"]` attribute to your crate (`{$crate_name}`)
+
+expand_malformed_feature_attribute =
+    malformed `feature` attribute input
+    .expected = expected just one word
+
+expand_remove_expr_not_supported =
+    removing an expression is not supported in this position
+
+expand_invalid_cfg_no_parens = `cfg` is not followed by parentheses
+expand_invalid_cfg_no_predicate = `cfg` predicate is not specified
+expand_invalid_cfg_multiple_predicates = multiple `cfg` predicates are specified
+expand_invalid_cfg_predicate_literal = `cfg` predicate key cannot be a literal
+expand_invalid_cfg_expected_syntax = expected syntax is
+
+expand_wrong_fragment_kind =
+    non-{$kind} macro in {$kind} position: {$name}
+
+expand_unsupported_key_value =
+    key-value macro attributes are not supported
+
+expand_incomplete_parse =
+    macro expansion ignores token `{$token}` and any following
+    .label = caused by the macro expansion here
+    .note = the usage of `{$macro_path}!` is likely invalid in {$kind_name} context
+    .suggestion_add_semi = you might be missing a semicolon here
+
+expand_remove_node_not_supported =
+    removing {$descr} is not supported in this position
+
+expand_module_circular =
+    circular modules: {$modules}
+
+expand_module_in_block =
+    cannot declare a non-inline module inside a block unless it has a path attribute
+    .note = maybe `use` the module `{$name}` instead of redeclaring it
+
+expand_module_file_not_found =
+    file not found for module `{$name}`
+    .help = to create the module `{$name}`, create file "{$default_path}" or "{$secondary_path}"
+
+expand_module_multiple_candidates =
+    file for module `{$name}` found at both "{$default_path}" and "{$secondary_path}"
+    .help = delete or rename one of them to remove the ambiguity