better try/catch fallback

compiler_tests.odin

@@ -1307,6 +1307,42 @@ main func() void {
 	testing.expect_value(t, len(diagnostics.items), 0)
 }
 
+@(test)
+catch_fallback_preserves_fallible_success_type :: proc(t: ^testing.T) {
+	text := `Read_Error :: enum {
+	failed
+}
+read func(value []u8, fail bool) []u8 ! Read_Error {
+	if fail {
+		return .failed
+	}
+	return value
+}
+take_slice func(value []u8) void { _ = value }
+main func() void {
+	expression :: read("ok", false) catch "fallback"
+	block :: read("ok", true) catch {
+		yield "fallback"
+	}
+	take_slice(expression)
+	take_slice(block)
+}
+`
+	source_file := source.Source{path="test.bro", text=text}
+	diagnostics := source.init_diagnostics(&source_file)
+	defer source.destroy_diagnostics(&diagnostics)
+	symbols := symbol.init_table()
+	defer symbol.destroy_table(&symbols)
+	stream := lexer.lex(&source_file, &diagnostics, &symbols)
+	defer delete(stream.items)
+	ast_module := parser.parse(&stream, &source_file, &diagnostics)
+	defer ast.destroy_module(&ast_module)
+	hir_module := checker.check(&ast_module, &diagnostics, &symbols)
+	defer hir.destroy_module(&hir_module)
+
+	testing.expect_value(t, len(diagnostics.items), 0)
+}
+
 @(test)
 pipeline_emits_specialized_calling_conventions_and_checked_add :: proc(t: ^testing.T) {
 	text := `sum_c c_func(a, b int) int {